Add checks to disallow use of isset( $var ) and empty()

[westonruter]

Feature Description

The use of empty() and isset( $var ) can mask code problems (e.g. typos). And they rarely need to be used, for example:

• https://www.beberlei.de/post/when_to_use_empty_in_php_i_say_never
• https://dev.to/aleksikauppila/using-isset-and-empty-hurts-your-code-aaa
• https://dev.to/klnjmm/never-use-empty-function-in-php-4pb0
• https://localheinz.com/articles/2023/05/10/avoiding-empty-in-php/

cf. comment by @felixarntz in https://github.com/WordPress/performance/pull/1091#discussion_r1543908524

We should consider adding sniffs to warn against their use. (Granted, PHPStan should catch the problematic uses of isset() and empty().)

[swissspidy]

https://github.com/phpstan/phpstan-strict-rules Is your friend.

Works like a charm

[westonruter]

But I said we were done with PHPStan PRs 😂

[felixarntz]

Per https://github.com/WordPress/performance/pull/1489#discussion_r1725404800, let's change this issue to be only about empty() - or, if we want to include isset(), it should only be prevented on variable checks like isset( $something ), but not on e.g. array key or object property checks like isset( $arr['something'] ).