process_fts_updates: Only DELETE if rows were processed.

puppet/zulip/files/postgresql/process_fts_updates

@@ -86,6 +86,7 @@ def update_fts_columns(conn: psycopg2.extensions.connection) -> int:
                 (message_id,),
             )
             ids.append(id)
+        if ids:
             cursor.execute("DELETE FROM fts_update_log WHERE id = ANY(%s)", (ids,))
         conn.commit()
         return len(ids)