Copy model docstring if no doc exists

Author: process0

graphene_pydantic/objecttype.py

@@ -86,7 +86,8 @@ def __init_subclass_with_meta__(
         if not registry:
             registry = get_global_registry(PydanticObjectType)
 
-        cls.__doc__ = model.__doc__
+        if not cls.__doc__:
+            cls.__doc__ = model.__doc__
 
         pydantic_fields = yank_fields_from_attrs(
             construct_fields(