Inhibit original callback from firing when sending initial changeset

osu.Game.Tests/Database/RealmSubscriptionRegistrationTests.cs

@@ -85,7 +85,7 @@ namespace osu.Game.Tests.Database
 
                 realm.Run(r => r.Refresh());
 
-                Assert.That(receivedChangesCount, Is.EqualTo(2));
+                Assert.That(receivedChangesCount, Is.EqualTo(1));
                 Assert.That(firstChanges, Is.Null);
 
                 registration.Dispose();

osu.Game/Database/RealmObjectExtensions.cs

@@ -302,7 +302,10 @@ namespace osu.Game.Database
                     // Realm might coalesce the initial callback, meaning we never receive a `ChangeSet` of `null` marking the first callback.
                     // Let's decouple it for simplicity in handling.
                     if (changes != null)
+                    {
                         callback(sender, null);
+                        return;
+                    }
                 }
 
                 callback(sender, changes);