Amend HBASE-14512 Cache UGI groups

Handle the case where tests might create a call with a null connection
diff --git a/hbase-server/src/main/java/org/apache/hadoop/hbase/ipc/RpcServer.java b/hbase-server/src/main/java/org/apache/hadoop/hbase/ipc/RpcServer.java
index 458a6b2..a9f5bf4 100644
--- a/hbase-server/src/main/java/org/apache/hadoop/hbase/ipc/RpcServer.java
+++ b/hbase-server/src/main/java/org/apache/hadoop/hbase/ipc/RpcServer.java
@@ -337,7 +337,7 @@
       this.isError = false;
       this.size = size;
       this.tinfo = tinfo;
-      this.user = connection.user;
+      this.user = connection == null ? null : connection.user;
       this.remoteAddress = remoteAddress;
     }