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/events/EventFactory.java

@@ -590,7 +590,7 @@ public class EventFactory {
     AccountAttribute who = new AccountAttribute();
     who.name = accountState.getAccount().getFullName();
     who.email = accountState.getAccount().getPreferredEmail();
-    who.username = accountState.getUserName();
+    who.username = accountState.getUserName().orElse(null);
     return who;
   }