DisplayMotd: Adjust to changed signature of CurrentUser#getUserName

The method now returns Optional<String>.

Change-Id: I1d0719b49df042d6f35892ed29275a323b8c1626

src/main/java/com/googlesource/gerrit/plugins/motd/DisplayMotd.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/motd/DisplayMotd.java b/src/main/java/com/googlesource/gerrit/plugins/motd/DisplayMotd.java
index 83912a2..de658f4 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/motd/DisplayMotd.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/motd/DisplayMotd.java
@@ -69,8 +69,8 @@
   }
 
   private String format(String message) {
-    if (user.getUserName() != null) {
-      message = message.replace("${user}", user.getUserName());
+    if (user.getUserName().isPresent()) {
+      message = message.replace("${user}", user.getUserName().get());
     }
     return message;
   }