Fix generating null return type from reflection in TypeGenerator

The TypeGenerator failed with "type 'null' cannot be nullable" when
`fromReflectionType()` was called for a method with a return type
"null" due to a incorrect comparison with the string "null". This
change corrects the comparison so that such methods are reflected
without errors.

Obvious fix.

Author: robertlemke

src/Generator/TypeGenerator.php

@@ -69,7 +69,7 @@ public static function fromReflectionType(
 
         return new self(
             $atomicType,
-            $atomicType->type !== 'mixed' && $atomicType !== 'null' && $type->allowsNull()
+            $atomicType->type !== 'mixed' && $atomicType->type !== 'null' && $type->allowsNull()
         );
     }