Make PushData.clientSecret mandatory.

Also do not restore the last session as a fallback, it can lead to error in a multi account context, or even when a ghost pusher send a Push.

libraries/pushproviders/api/src/main/kotlin/io/element/android/libraries/pushproviders/api/PushData.kt

@@ -22,5 +22,5 @@ data class PushData(
     val eventId: EventId,
     val roomId: RoomId,
     val unread: Int?,
-    val clientSecret: String?,
+    val clientSecret: String,
 )