feat: add explicit null check in XMLSchemaTest (PHPStan Level 8)

[shakaran]

Improves the robustness of the testValidateSchema method in XMLSchemaTest.php by adding null checks before importing XML nodes. This prevents potential errors when the expected XML elements are not present.

Test robustness improvements:

• Added checks to ensure that dbal and orm elements are not null before attempting to import them with importNode, preventing possible exceptions if these elements are missing.

 vendor/bin/phpstan analyze --level=8 tests/DependencyInjection/XMLSchemaTest.php
 ------ ------------------------------------------------------------------------------------------------ 
  Line   tests/DependencyInjection/XMLSchemaTest.php                                                     
 ------ ------------------------------------------------------------------------------------------------ 
  :45    Parameter #1 $node of method DOMDocument::importNode() expects DOMNode, DOMElement|null given.  
         🪪  argument.type                                                                               
  :58    Parameter #1 $node of method DOMDocument::importNode() expects DOMNode, DOMElement|null given.  
         🪪  argument.type                                                                               
 ------ ------------------------------------------------------------------------------------------------ 

[ostrolucky]

We are not going with PR by file approach. It's going to be a PR by raised phpstan level.