Improve support for is_file and family

[villfa]

Feature request

Correct me if I'm wrong but I think the functions that expect a valid path (file_exists, is_readable, ...) always return false when an empty string is provided ( https://3v4l.org/TcecN ).
So when those functions return true we can say that the argument is a non-empty string.

If my assumption is right it could help with situations like this:
https://phpstan.org/r/e9812010-b816-4a57-8fb1-036c6a6b177e

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