Fix `phpstan`-errors instead of adding exceptions for the rules

[mattagohni]

The following exception to the phpstan-rules should be reolved in the code.

  # Symfony null typehints
        - '/Cannot call method arrayNode\(\) on **Symfony\\Component\\Config\\Definition\\Builder\\NodeParentInterface\|null/'**

Affected Code:

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  Line   Fixture/ProductBundleFixture.php                                       
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  25     Cannot call method arrayNode() on                                      
         Symfony\Component\Config\Definition\Builder\NodeParentInterface|null.  
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  Line   Form/Type/ProductBundleSlotType.php         
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  67     Cannot call method getData() on             
         Symfony\Component\Form\FormInterface|null.  
  67     Cannot call method getParent() on           
         Symfony\Component\Form\FormInterface|null.  
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