[FLINK-36876] add tag

Author: chenyuzhi

flink-clients/src/main/java/org/apache/flink/client/program/rest/RestClusterClient.java

@@ -250,7 +250,7 @@ private RestClusterClient(
             T clusterId,
             WaitStrategy waitStrategy,
             ClientHighAvailabilityServicesFactory clientHAServicesFactory,
-            EventLoopGroup group)
+            @Nullable EventLoopGroup group)
             throws Exception {
         this.configuration = checkNotNull(configuration);