CurrentUser#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: I97e33caa8e9cc8092dc398e3b383d7afd5beb96f
Signed-off-by: Edwin Kempin <ekempin@google.com>

java/com/google/gerrit/server/CurrentUser.java

@@ -19,6 +19,7 @@ import com.google.gerrit.reviewdb.client.Account;
 import com.google.gerrit.server.account.GroupMembership;
 import com.google.gerrit.server.account.externalids.ExternalId;
 import com.google.inject.servlet.RequestScoped;
+import java.util.Optional;
 import java.util.function.Consumer;
 
 /**
@@ -90,8 +91,8 @@ public abstract class CurrentUser {
   public abstract GroupMembership getEffectiveGroups();
 
   /** Unique name of the user on this server, if one has been assigned. */
-  public String getUserName() {
-    return null;
+  public Optional<String> getUserName() {
+    return Optional.empty();
   }
 
   /** Check if user is the IdentifiedUser */