engine/db: Return None in case no objects are found.

This helps for consistency in responses and error handling

Signed-off-by: ZorEl212 <[email protected]>

models/engine/mongodb.py

@@ -31,7 +31,7 @@ def process_collection(collection):
                 collection = self.database[cls.get_cls_name()]
                 process_collection(collection)
 
-        return docs
+        return docs if len(docs) > 0 else None
 
     def new(self, obj):
         collection = self.database[obj.cls_name]