message: fix hadoop_file_system's bug when not provide namenode info
diff: @@ -155,7 +155,7 @@ Status HadoopFileSystem::Connect(StringPiece fname, hdfsFS* fs) {
  StringPiece defaultScheme, defaultCluster, defaultPath;
  io::ParseURI(defaultFS, &defaultScheme, &defaultCluster, &defaultPath);
  - if (scheme != defaultScheme || namenode != defaultCluster) {
  + if (scheme != defaultScheme || (namenode != "" && namenode != defaultCluster)) {
  return errors::Unimplemented(
  "viewfs is only supported as a fs.defaultFS.");
  }
  @@ -164,7 +164,7 @@ Status HadoopFileSystem::Connect(StringPiece fname, hdfsFS* fs) {
  // https://github.com/tensorflow/tensorflow/blob/v1.0.0/third_party/hadoop/hdfs.h#L259
  hdfs_->hdfsBuilderSetNameNode(builder, "default");
  } else {
  - hdfs_->hdfsBuilderSetNameNode(builder, nn.c_str());
  + hdfs_->hdfsBuilderSetNameNode(builder, nn == "" ? "default" : nn.c_str());
  }
  // KERB_TICKET_CACHE_PATH will be deleted in the future, Because KRB5CCNAME is
  // the build in environment variable of Kerberos, so KERB_TICKET_CACHE_PATH
  