Fix PHPStan errors in tests

[westonruter]

Summary

This fixes two PHPStan errors:

 ------ ---------------------------------------------- 
  Line   tests/admin/load-tests.php  
 ------ ---------------------------------------------- 
  94     Filter callback return statement is missing.  
 ------ ---------------------------------------------- 

The __return_empty_array is the appropriate callback for the perflab_active_modules filter, not __return_null.

 ------ ---------------------------------------------------- 
  Line   tests/modules/images/webp-uploads/helper-tests.php  
 ------ ---------------------------------------------------- 
  367    Filter callback return statement is missing.        
 ------ ---------------------------------------------------- 

In this case, the desire is to test when an invalid return value is supplied for the webp_uploads_upload_image_mime_transforms filter, so in this case the PHPStan error is ignored.

Checklist

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[westonruter]

Strange. This is reported as an error for me locally but it isn't in GHA?

[westonruter]

It's also an error being reported in #898, but not in trunk.

[mukeshpanchal27]

Strange issue 🤷🏻‍♂️

[felixarntz]

Is it maybe because of a different PHPStan strictness or other config? Or because of a different PHP version (which could lead to a diffferent PHPStan version)? 🤔

[westonruter]

Looks like a composer update fixed it!

[westonruter]

On trunk currently if I try doing composer install, I get an error:

Verifying lock file contents can be installed on current platform.
Your lock file does not contain a compatible set of packages. Please run composer update.

  Problem 1
    - doctrine/instantiator is locked to version 2.0.0 and an update of this package was not requested.
    - doctrine/instantiator 2.0.0 requires php ^8.1 -> your php version (8.0.30) does not satisfy that requirement.
  Problem 2
    - doctrine/instantiator 2.0.0 requires php ^8.1 -> your php version (8.0.30) does not satisfy that requirement.
    - phpunit/phpunit 9.6.13 requires doctrine/instantiator ^1.3.1 || ^2 -> satisfiable by doctrine/instantiator[2.0.0].
    - phpunit/phpunit is locked to version 9.6.13 and an update of this package was not requested.

So that's why I did composer update.

[westonruter]

However, in 5c009ba @felixarntz observed that doctrine/instantiator was getting downgraded, which shouldn't be happening. So I upgraded from PHP 8.0 to 8.1 and re-ran composer update and now this is no longer happening.