FIX Address the case of no volumes to scrub when given the scrubbing strategy

[htwangtw]

A data set can have data where non steady volumes were not present and no volumes scrubbed.

This was detected from running on ds000228.

Bug introduced by #144

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.03%. Comparing base (e44fca6) to head (6a71292).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #156      +/-   ##
==========================================
- Coverage   94.05%   94.03%   -0.03%     
==========================================
  Files          18       18              
  Lines         757      754       -3     
==========================================
- Hits          712      709       -3     
  Misses         45       45              

Files with missing lines	Coverage Δ
giga_connectome/denoise.py	86.20% <100.00%> (-0.46%)	⬇️
giga_connectome/tests/test_denoise.py	100.00% <ø> (ø)