[TypeDeclaration] Ensure ArrayType on ReturnTypeFromStrictNewArrayRector::shouldAddReturnArrayDocType()

[samsonasik]

Avoid error on unioned array:

         "System error:                                                                                                 
         "Rector\TypeDeclaration\Rector\ClassMethod\ReturnTypeFromStrictNewArrayRector::shouldAddReturnArrayDocType():  
         Argument #1 ($arrayType) must be of type PHPStan\Type\ArrayType, PHPStan\Type\UnionType given,