SidetagOwnerTest: fix none value of tag/fromtag as well

plugins/hub/sidetag_hub.py

@@ -85,7 +85,9 @@ class SidetagOwnerTest(koji.policy.MatchTest):
         for field in fields:
             if field not in data:
                 return False
-            tag = get_tag(data[field])
+            tag = data.get(field)
+            if tag:
+                tag = get_tag(tag)
             if not tag or not is_sidetag_owner(tag, user):
                 return False
         return True