hub: return empty list in get_notification_recipients()

If we find no recipients, return an empty list, rather than None.

This fixes a crash in build_notifications() where we expect to call
len() on the return value from get_notification_recipients().

Fixes: https://pagure.io/koji/issue/2307

hub/kojihub.py

@@ -8383,7 +8383,7 @@ def get_notification_recipients(build, tag_id, state):
         #   using all package owners from all tags would be way overkill.
 
     if not recipients:
-        return None
+        return []
 
     # apply the out outs
     user_ids = list(set([r['user_id'] for r in recipients]))