Merge topic 'source_group-TREE-no-FILES'

87788353cc source_group: Add test/example for TREE without FILES
d85238a2f2 source_group: Fix TREE without FILES

Acked-by: Kitware Robot <kwrobot@kitware.com>
Tested-by: buildbot <buildbot@kitware.com>
Merge-request: !8599