Fix _get_entity_pair could receive None as input

telethon/utils.py

@@ -583,6 +583,9 @@ def _get_entity_pair(entity_id, entities, cache,
     """
     Returns ``(entity, input_entity)`` for the given entity ID.
     """
+    if not entity_id:
+        return None, None
+
     entity = entities.get(entity_id)
     try:
         input_entity = cache.get(resolve_id(entity_id)[0])._as_input_peer()