Proper handling of empty folder

[xinaxu]

Empty folder from supported backends will now have a database entry.

resolves #220

[codecov]

Codecov Report

Patch coverage: 51.42% and project coverage change: -0.01% ⚠️

Comparison is base (d0a6c36) 73.93% compared to head (fc3bbdd) 73.92%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #328      +/-   ##
==========================================
- Coverage   73.93%   73.92%   -0.01%     
==========================================
  Files         136      136              
  Lines        8533     8561      +28     
==========================================
+ Hits         6309     6329      +20     
- Misses       1565     1570       +5     
- Partials      659      662       +3     

Files Changed	Coverage Δ
storagesystem/types.go	96.10% <ø> (ø)
storagesystem/rclone.go	85.29% <50.00%> (-2.29%)	⬇️
scan/scan.go	58.94% <52.63%> (-1.58%)	⬇️

... and 2 files with indirect coverage changes

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