schema: fix topic command output

Return topic names as text instead of binary blob.

This fixes ipa help topic display.

https://fedorahosted.org/freeipa/ticket/4739

Reviewed-By: David Kupka <dkupka@redhat.com>

ipaclient/remote_plugins/schema.py

@@ -237,6 +237,8 @@ def _create_topic(schema):
         topic['doc'] = ConcatenatedLazyText(schema['doc'])
     if 'topic_topic' in schema:
         topic['topic'] = str(schema['topic_topic'])
+    else:
+        topic['topic'] = None
 
     return topic