Fixed return value when subtags query fails.

ccnode/node.py

@@ -213,7 +213,7 @@ class MainLoop(object):
         if sub_db is None:
             # FIXME should we also raise here ?
             logger.debug('Failed to find sub_id %s', sub_id)
-            return
+            return {}
 
         return get_tags(sub_db, tags, noresolve_tags)
     # End RPC handlers definitions