Fixed hang when submitting mappings and no changes are made

Added missing listener call before return Closes elastic#3381

Author: javanna

src/main/java/org/elasticsearch/cluster/metadata/MetaDataMappingService.java

@@ -452,6 +452,8 @@ public ClusterState execute(final ClusterState currentState) throws Exception {
  }
 
  if (mappings.isEmpty()) {
+ // no changes, return
+ listener.onResponse(new Response(true));
  return currentState;
  }