source_group: Restore behavior of TREE with empty FILES
Since !8599 (merged) we incorrectly treat a FILES argument with no values as if it were not given at all.
Fixes: #25353 (closed)
Backport: release
Since !8599 (merged) we incorrectly treat a FILES argument with no values as if it were not given at all.
Fixes: #25353 (closed)
Backport: release