E.g.: `https://lnk.lt/tiesiogiai` would previously raise
"[plugins.lnk][error] Unknown channel: None".
This is because, per Python's docs, matchdict:
> Return a dictionary containing all the named subgroups of the match,
> keyed by the subgroup name. The default argument is used for groups
> that did not participate in the match; it defaults to None.
Therefore, get("channel", "lnk") would return the value of `None`,
as the key exists, rather than falling back to `"lnk"`.

Billy2011 pushed a commit to Billy2011/streamlink-27 that referenced this pull request

Co-Authored-By: bastimeyer <mail@bastimeyer.de>