filter_var() should return non-empty-string if argument is non-empty-string

# Bug report

### Code snippet that reproduces the problem

https://phpstan.org/r/3cbf9f66-1f2f-4a7a-9459-afc5f1da627a

If I've already validated the string is non-empty, then `filter_var()` is either going to return a non-empty-string, or false.

### Expected output

Should not report an error.
