default void preListNamespaces(ObserverContext<MasterCoprocessorEnvironment> ctx,
    List<String> namespaces) throws IOException {
  }
Called before a listNamespaces request has been processed.
@param ctx        the environment to interact with the framework and master
@param namespaces an empty list, can be filled with what to return if bypassing
@throws IOException if something went wrong
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/MasterObserver.java