Fix: unexpected querying all files when deleting an empty folder

[topjohncian]

Fix deleting an empty folder takes a too long time when the size of files is large.

Empty folders have no children, so the parameter of RemoveFilesWithSoftLinks is also an empty slice. Then, it builds and executes a query SELECT * FROM files with no WHERE conditions, which takes the longest time.

[HFO4]

Nice catch, thanks!