57eb8539fc
In some cases, we can have nodegroups without attached nodetree, this case is checked in top `node_group_has_output()` entry point, but not in recursive `node_group_has_output_dfs()`. Issue appeared after rB42824f4403db3a35, presumably because check on node flag itself was skipping that case, which is no more possible when using ID tags. Reported by sebastian_k over IRC, thanks.