Fix PHPStan failure when getting object value from JsonParseNode

[Ndiritu]

Using the generic PHPDoc tags added in microsoft/kiota-abstractions-php#42 the expected return type after calling the Parsable factory method is an object that implements Parsable

PHPStan currently fails with:

------ -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 
  Line   JsonParseNode.php                                                                                                                                                                                                         
 ------ -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 
  :116   Method Microsoft\Kiota\Serialization\Json\JsonParseNode::getObjectValue() should return (T of Microsoft\Kiota\Abstractions\Serialization\Parsable)|null but returns Microsoft\Kiota\Abstractions\Serialization\Parsable.  
         💡 Type Microsoft\Kiota\Abstractions\Serialization\Parsable is not always the same as T. It breaks the contract for some argument types, typically subtypes.                                                               
 ------ --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

[sonarqubecloud]

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