Fix 'Conversation label cannot be empty' error (#6823)

This happens when building a `ShortcutInfoCompat` in `DefaultNotificationConversationService.onSendMessage` when the provided room name is not null but it's empty.

libraries/push/api/src/main/kotlin/io/element/android/libraries/push/api/notifications/conversations/NotificationConversationService.kt

@@ -22,7 +22,7 @@ interface NotificationConversationService {
     suspend fun onSendMessage(
         sessionId: SessionId,
         roomId: RoomId,
-        roomName: String,
+        roomName: String?,
         roomIsDirect: Boolean,
         roomAvatarUrl: String?,
     )