AccountState#getUserName: Return Optional<String> instead of nullable String

This makes it more explicit that callers must handle the case where the
returned username is absent.

Change-Id: I01d2e3fd32726db6175c157bc71a551d35b4803f
Signed-off-by: Edwin Kempin <ekempin@google.com>

java/com/google/gerrit/server/extensions/events/EventUtil.java

@@ -102,7 +102,7 @@ public class EventUtil {
     AccountInfo accountInfo = new AccountInfo(account.getId().get());
     accountInfo.email = account.getPreferredEmail();
     accountInfo.name = account.getFullName();
-    accountInfo.username = accountState.getUserName();
+    accountInfo.username = accountState.getUserName().orElse(null);
     return accountInfo;
   }