Check VAPID key to avoid VapidNotValidException

Some server softwares like [Hollo](https://github.com/fedify-dev/hollo/) return an empty non-null vapid key which lead to a crash. It should inform user instead of crashing I received this crash report: ``` 2.3.0+fork.110.moshinda (110) 2025-11-09T16:51:56.460Z java.lang.RuntimeException: java.lang.reflect.InvocationTargetException at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:641) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:978) Caused by: java.lang.reflect.InvocationTargetException at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:631) ... 1 more Caused by: org.unifiedpush.android.connector.UnifiedPush$VapidNotValidException at org.unifiedpush.android.connector.UnifiedPush.register(UnifiedPush.kt:252) at org.unifiedpush.android.connector.UnifiedPush.register(UnifiedPush.kt:228) at org.unifiedpush.android.connector.UnifiedPush.register(UnifiedPush.kt:200) at org.joinmastodon.android.utils.UnifiedPushHelper.registerAllAccounts(UnifiedPushHelper.java:49) at org.joinmastodon.android.fragments.settings.SettingsNotificationsFragment.lambda$showUnifiedPushRegisterDialog$28(SettingsNotificationsFragment.java:367) at org.joinmastodon.android.fragments.settings.SettingsNotificationsFragment.$r8$lambda$gvxT_BzhJleOu10rYUVJ1mJv0dk(SettingsNotificationsFragment.java:0) at org.joinmastodon.android.fragments.settings.SettingsNotificationsFragment$$ExternalSyntheticLambda19.onClick(R8$$SyntheticClass:0) at com.android.internal.app.AlertController$AlertParams$3.onItemClick(AlertController.java:1250) at android.widget.AdapterView.performItemClick(AdapterView.java:330) at android.widget.AbsListView.performItemClick(AbsListView.java:1210) at android.widget.AbsListView$PerformClick.run(AbsListView.java:3202) at android.os.Handler.handleCallback(Handler.java:938) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:233) at android.app.ActivityThread.main(ActivityThread.java:8068) ... 3 more ```