Amend HBASE-14512 Cache UGI groups

Handle the case where tests might create a call with a null connection

hbase-server/src/main/java/org/apache/hadoop/hbase/ipc/RpcServer.java

@@ -339,7 +339,7 @@ public class RpcServer implements RpcServerInterface, ConfigurationObserver {
       this.isError = false;
       this.size = size;
       this.tinfo = tinfo;
-      this.user = connection.user;
+      this.user = connection == null ? null : connection.user;
       this.remoteAddress = remoteAddress;
       this.retryImmediatelySupported = connection.retryImmediatelySupported;
     }