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/test/src/main/kotlin/io/element/android/libraries/push/test/notifications/conversations/FakeNotificationConversationService.kt

@@ -16,7 +16,7 @@ class FakeNotificationConversationService : NotificationConversationService {
     override suspend fun onSendMessage(
         sessionId: SessionId,
         roomId: RoomId,
-        roomName: String,
+        roomName: String?,
         roomIsDirect: Boolean,
         roomAvatarUrl: String?,
     ) = Unit