Ensure realm subscriptions always fire initial callback with null `Ch…

…angeSet`

We expect this to be the case, but it turns out that it [may be
coalesced](https://www.mongodb.com/docs/realm-sdks/dotnet/latest/reference/Realms.IRealmCollection-1.html#Realms_IRealmCollection_1_SubscribeForNotifications_Realms_NotificationCallbackDelegate__0__Realms_KeyPathsCollection_):

> Notifications are delivered via the standard event loop, and so can't
> be delivered while the event loop is blocked by other activity. When
> notifications can't be delivered instantly, multiple notifications may
> be coalesced into a single notification. This can include the
> notification with the initial collection.

Rather than struggle with handling this locally every time, let's fix
the callback at our end to ensure we receive the initial null case.

I've raised concern for the API being a bit silly with realm
(realm/realm-dotnet#3641).

osu.Game/Database/RealmObjectExtensions.cs

@@ -65,7 +65,8 @@ public static class RealmObjectExtensions
                          if (!d.Beatmaps.Contains(existingBeatmap))
                          {
                              Debug.Fail("Beatmaps should never become detached under normal circumstances. If this ever triggers, it should be investigated further.");
-                             Logger.Log("WARNING: One of the difficulties in a beatmap was detached from its set. Please save a copy of logs and report this to devs.", LoggingTarget.Database, LogLevel.Important);
+                             Logger.Log("WARNING: One of the difficulties in a beatmap was detached from its set. Please save a copy of logs and report this to devs.", LoggingTarget.Database,
+                                 LogLevel.Important);
                              d.Beatmaps.Add(existingBeatmap);
                          }
 
@@ -291,7 +292,21 @@ public static IDisposable QueryAsyncWithNotifications<T>(this IRealmCollection<T
             if (!RealmAccess.CurrentThreadSubscriptionsAllowed)
                 throw new InvalidOperationException($"Make sure to call {nameof(RealmAccess)}.{nameof(RealmAccess.RegisterForNotifications)}");
 
-            return collection.SubscribeForNotifications(callback);
+            bool initial = true;
+            return collection.SubscribeForNotifications(((sender, changes) =>
+            {
+                if (initial)
+                {
+                    initial = false;
+
+                    // 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);
+                }
+
+                callback(sender, changes);
+            }));
         }
 
         /// <summary>