silently succeed when adding the same serialized file in Python

PiperOrigin-RevId: 477321377

python/descriptor_pool.c

@@ -218,8 +218,7 @@ static PyObject* PyUpb_DescriptorPool_DoAddSerializedFile(
     }
     const upb_MessageDef* m = PyUpb_DescriptorPool_GetFileProtoDef();
     if (upb_Message_IsEqual(proto, existing, m)) {
-      Py_INCREF(Py_None);
-      result = Py_None;
+      result = PyUpb_FileDescriptor_Get(file);
       goto done;
     }
   }