Fix: Add a check for usage of the empty() construct

[RikudouSage]

Description

I've noticed some code which uses the empty() construct. The construct does way too many things and should not be used in any modern code. Additionally it looks like a function but isn't, which might cause problems in a few cases.

This PR adds a PHPStan rule that checks for usage of empty() and offers a hint if the type of variable is known.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[RikudouSage]

You can check what the output looks like here: https://github.com/Unleash/unleash-client-php/actions/runs/6997953019/job/19035484861?pr=188

[sighphyre]

Oh wow, that's pretty cool. Yes please!