obs-outputs: Add multitrack flag to null output

plugins/obs-outputs/null-output.c

@@ -89,7 +89,7 @@ static void null_output_data(void *data, struct encoder_packet *packet)
 
 struct obs_output_info null_output_info = {
 	.id = "null_output",
-	.flags = OBS_OUTPUT_AV | OBS_OUTPUT_ENCODED,
+	.flags = OBS_OUTPUT_AV | OBS_OUTPUT_ENCODED | OBS_OUTPUT_MULTI_TRACK_AV,
 	.get_name = null_output_getname,
 	.create = null_output_create,
 	.destroy = null_output_destroy,