Describe the bug

I'm working on to generate the documentation for my library ineqpy which use numpydoc as docstring-format. When I run the command mkdocs build fails with the following traceback (which is hard to debug):

IneqPy docs/add-mkdocs* (15:15|77s) $ pipenv run mkdocs build && pipenv run mkdocs serve
INFO     -  Cleaning site directory
INFO     -  Building documentation to directory: /home/mgreco/github/mmngreco/IneqPy/site
ERROR    -  Error reading page 'index.md': 'NoneType' object has no attribute 'strip'
Traceback (most recent call last):
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/bin/mkdocs", line 8, in <module>
    sys.exit(cli())
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/click/core.py", line 1128, in __call__
    return self.main(*args, **kwargs)
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/click/core.py", line 1053, in main
    rv = self.invoke(ctx)
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/click/core.py", line 1659, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/click/core.py", line 1395, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/click/core.py", line 754, in invoke
    return __callback(*args, **kwargs)
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/mkdocs/__main__.py", line 187, in build_command
    build.build(config.load_config(**kwargs), dirty=not clean)
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/mkdocs/commands/build.py", line 292, in build
    _populate_page(file.page, config, files, dirty)
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/mkdocs/commands/build.py", line 174, in _populate_page
    page.render(config, files)
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/mkdocs/structure/pages.py", line 174, in render
    self.content = md.convert(self.markdown)
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/markdown/core.py", line 264, in convert
    root = self.parser.parseDocument(self.lines).getroot()
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/markdown/blockparser.py", line 90, in parseDocument
    self.parseChunk(self.root, '\n'.join(lines))
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/markdown/blockparser.py", line 105, in parseChunk
    self.parseBlocks(parent, text.split('\n\n'))
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/markdown/blockparser.py", line 123, in parseBlocks
    if processor.run(parent, blocks) is not False:
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/mkdocstrings/extension.py", line 120, in run
    html, handler, data = self._process_block(identifier, block, heading_level)
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/mkdocstrings/extension.py", line 199, in _process_block
    rendered = handler.renderer.render(data, rendering)
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/mkdocstrings/handlers/python.py", line 96, in render
    return template.render(
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/jinja2/environment.py", line 1291, in render
    self.environment.handle_exception()
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/jinja2/environment.py", line 925, in handle_exception
    raise rewrite_traceback_stack(source=source)
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/mkdocstrings/templates/python/material/module.html", line 53, in top-level templ
ate code
    {% include "children.html" with context %}
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/mkdocstrings/templates/python/material/children.html", line 30, in top-level tem
plate code
    {% include "class.html" with context %}
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/mkdocstrings/templates/python/material/class.html", line 54, in top-level templa
te code
    {% include "docstring.html" with context %}
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/mkdocstrings/templates/python/material/docstring.html", line 8, in top-level tem
plate code
    {% include "attributes.html" with context %}
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/mkdocstrings/templates/python/material/attributes.html", line 16, in top-level t
emplate code
    <td>{{ attribute.description|convert_markdown(heading_level, html_id) }}</td>
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/mkdocstrings/handlers/base.py", line 158, in do_convert_markdown
    return Markup(self._md.convert(text))
  File "/home/mgreco/.local/share/virtualenvs/IneqPy-eI7PkriJ/lib/python3.8/site-packages/markdown/core.py", line 248, in convert
    if not source.strip():
AttributeError: 'NoneType' object has no attribute 'strip'

To Reproduce

Fortunately, I could isolate the issue or I think I do it:

Create a file with the following content (I called module for debugging reasons) :

Here's how to document a class using numpy-style

# module.py
class Survey:
    """Survey it's a data structure that handles survey data.

    Attributes
    ----------
    df : pandas.DataFrame
        Data source.

    Methods
    -------
    foo(a=None, b=0.5)
        foo method.
    """
    def __init__(self, df) -> None:
        self.df = df
    def foo(self, a=None, b=0.5):
        pass

Here's my mkdocs.yml:

site_name: IneqPy's Documentation
theme: material
plugins:
- mkdocstrings:
    default_handler: python
    handlers:
      python:
        selection:
          docstring_style: numpy
        rendering:
          show_source: true
    watch:
      - src/ineqpy

Here's the index.md

# Welcome to IneqPy's documentation!
# Reference
::: ineqpy.module

Execute mkdocs build and see the error.

Information (please complete the following information):

OS: Ubuntu 20.04
mkdocstrings version: 0.17.0

mkdocs                        1.2.3
mkdocs-autorefs               0.3.1
mkdocs-material               8.1.8
mkdocs-material-extensions    1.0.3
mkdocstrings                  0.17.0

Additional context

This only happens using numpy doc style. I've tried google style and the build works but the html has some problems.