\Psl\Filesystem\get_extension() can return empty string

[dragosprotung]

Describe the bug
\Psl\Filesystem\get_extension() can return empty string

To Reproduce

var_dump(\Psl\Filesystem\get_extension('test.')); // string(0) ""

Expected behavior
null to be returned.

Additional context
Not sure how this should be fixed.
One aproach to fix this would be to change the return from non-empty-string|null to string|null.
Personally I would expect null to be returned for a file without an extension but with a . in the name (like in the example above).

I can make a PR once the direction is decided.

[azjezz]

I'm leaning more toward returning null in this case.

[dragosprotung]

Closed by #350