This tries to open the given file to use as stdin, and if it fails,
for any reason, it uses /dev/null instead.
This is useful in cases where we would otherwise do either of these:
```fish
test -r /path/to/file
and string match foo < /path/to/file
cat /path/to/file 2>/dev/null | string match foo
```
This both makes it nicer and shorter, *and* helps with TOCTTOU - what if the file is removed/changed after the check?
The reason for reading /dev/null instead of a closed fd is that a closed fd will often cause an error.
In case opening /dev/null fails, it still skips the command.
That's really a last resort for when the operating system
has turned out to be a platypus and not a unix.
Fixes fish-shell#4865

This is mostly used instead of `test -r foo && string match bar <foo`
or `set bar (cat foo 2>/dev/null)`.
In doing so it can avoid a weird race condition where the file changes
between the check and the use, and it can avoid an external process or
having to open the file twice.
However it can also cause more work to happen - if you do
```fish
if test -r file
   # do a lot of work with file
end
```
it might be prudent to keep doing that. In the cases here it's usually
just a few `string` calls, which would operate on nothing and so be
pretty quick.

faho deleted the try-input-redirection branch