Fix notification crashes caused by unknown notification type

[kyori19]

Thanks for developing this elegant app!
I encountered a crash caused by new sign up notifications so I made a quick fix for that.
If you think there's better way to fix this problem, feel free to close this PR.

E/AndroidRuntime: FATAL EXCEPTION: databaseThread
    Process: org.joinmastodon.android, PID: 1642
    java.lang.NullPointerException: Attempt to invoke virtual method 'int org.joinmastodon.android.model.Notification$Type.ordinal()' on a null object reference
        at org.joinmastodon.android.api.CacheController.lambda$putNotifications$9(CacheController.java:200)
        at org.joinmastodon.android.api.CacheController$$ExternalSyntheticLambda5.run(Unknown Source:6)
        at org.joinmastodon.android.api.CacheController.lambda$runOnDbThread$14(CacheController.java:268)
        at org.joinmastodon.android.api.CacheController.$r8$lambda$s8ETJe7YHvU35-CsGtQJHgulQC0(Unknown Source:0)
        at org.joinmastodon.android.api.CacheController$$ExternalSyntheticLambda12.run(Unknown Source:6)
        at android.os.Handler.handleCallback(Handler.java:938)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:223)
        at me.grishka.appkit.utils.WorkerThread.run(WorkerThread.java:54)

[grishka]

Thanks!