Fix blocking/stack trace with empty list (#4191)

homeassistant/helpers/entity_component.py

@@ -296,6 +296,10 @@ class EntityPlatform(object):
         tasks = [self._async_process_entity(entity, update_before_add)
                  for entity in new_entities]
 
+        # handle empty list from component/platform
+        if not tasks:
+            return
+
         yield from asyncio.gather(*tasks, loop=self.component.hass.loop)
         yield from self.component.async_update_group()