Allow null users to still be displayed in the participant list

The fix here is correcting the access of `user.Country`. The deicision
to have null users display is because this is the best we can do (if
osu-web could not resolve the user). We still want the users in the
lobby to be aware of this user's presence, rather than hiding them from
view.

osu-stable does a similar thing, showing these users as `[Loading]`. I
decided to go with blank names instead because having *any* text there
causes confusion. We can iterate on this in future design updates.

osu.Game.Tests/Visual/Multiplayer/TestSceneMultiplayerParticipantsList.cs

@@ -43,6 +43,16 @@ namespace osu.Game.Tests.Visual.Multiplayer
             AddAssert("two unique panels", () => this.ChildrenOfType<ParticipantPanel>().Select(p => p.User).Distinct().Count() == 2);
         }
 
+        [Test]
+        public void TestAddNullUser()
+        {
+            AddAssert("one unique panel", () => this.ChildrenOfType<ParticipantPanel>().Select(p => p.User).Distinct().Count() == 1);
+
+            AddStep("add non-resolvable user", () => Client.AddNullUser(-3));
+
+            AddUntilStep("two unique panels", () => this.ChildrenOfType<ParticipantPanel>().Select(p => p.User).Distinct().Count() == 2);
+        }
+
         [Test]
         public void TestRemoveUser()
         {