Document ValueError for empty filename in getimagesize()

[lacatoire]

Summary

Document the behavior change in PHP 8.0.0 where getimagesize() now throws a ValueError when an empty string is passed as the filename parameter.

Previously, passing an empty filename would emit an E_WARNING and return false. As of PHP 8.0.0, a ValueError is thrown instead.

Changes:

• Add documentation in the Errors section
• Add changelog entry for PHP 8.0.0

Fixes #3714