default void preListNamespaceDescriptors(ObserverContext<MasterCoprocessorEnvironment> ctx,
    List<NamespaceDescriptor> descriptors) throws IOException {
  }
Called before a listNamespaceDescriptors request has been processed.
@param ctx         the environment to interact with the framework and master
@param descriptors an empty list, can be filled with what to return by coprocessor
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/MasterObserver.java