[Federation] Fix DefaultRequestInterceptorREST Client NPE

@Override
public void init(String user) {
  webAppAddress = WebAppUtils.getRMWebAppURLWithScheme(getConf());
  client = RouterWebServiceUtil.createJerseyClient(getConf());
} 

private DefaultRequestInterceptorREST createInterceptorForSubCluster(
    SubClusterId subClusterId, String webAppAddress) {

  final Configuration conf = this.getConf();

  String interceptorClassName = conf.get(
      YarnConfiguration.ROUTER_WEBAPP_DEFAULT_INTERCEPTOR_CLASS,
      YarnConfiguration.DEFAULT_ROUTER_WEBAPP_DEFAULT_INTERCEPTOR_CLASS);
  DefaultRequestInterceptorREST interceptorInstance = null;
  try {
    Class<?> interceptorClass = conf.getClassByName(interceptorClassName);
    if (DefaultRequestInterceptorREST.class
        .isAssignableFrom(interceptorClass)) {
      interceptorInstance = (DefaultRequestInterceptorREST) ReflectionUtils
          .newInstance(interceptorClass, conf);

    } else {
      throw new YarnRuntimeException(
          "Class: " + interceptorClassName + " not instance of "
              + DefaultRequestInterceptorREST.class.getCanonicalName());
    }
  } catch (ClassNotFoundException e) {
    throw new YarnRuntimeException(
        "Could not instantiate ApplicationMasterRequestInterceptor: "
            + interceptorClassName,
        e);
  }

  String webAppAddresswithScheme =
      WebAppUtils.getHttpSchemePrefix(this.getConf()) + webAppAddress;
  interceptorInstance.setWebAppAddress(webAppAddresswithScheme);
  interceptorInstance.setSubClusterId(subClusterId);
  // Missing initialization DefaultRequestInterceptorREST#Client Code  
  interceptors.put(subClusterId, interceptorInstance);
  return interceptorInstance;
}