fix: return real used_space/total_space for get_group_file_system_info (#1944)

The action hardcoded used_space=0 and total_space=10GiB. The kernel's
getGroupFileList call already returns the group's space usage in its
groupSpaceResult, so read it (a single-item fetch) instead of the
placeholders. Falls back to the previous constants when the field is
absent, so there is no regression.

Also relax the empty-count guard from `!groupFileCount` to `=== undefined`
so a group with zero files is not mistaken for "Group not found".