Closes https://github.com/ppy/osu/issues/25815.
`CollectionDropdown.collectionsChanged()` was assuming that if it
received `null` changes, then it must mean that the change subscription
is being initialised and the `filters` list will not contain any items.
However, that is not the only circumstance wherein a realm subscription
can fire with `null` changes; that can also happen after the main realm
instance gets recycled via the notification registration flow:
2f28a92f0a/osu.Game/Database/RealmAccess.cs (L545-L549)2f28a92f0a/osu.Game/Database/RealmAccess.cs (L1228-L1251)
Therefore, to fix the crash, just ensure that the list is cleared every
time.
