DM-35752: Allow None dataset ref to be passed to ButlerQC

[andy-slac]

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

Merging #269 (7562f1f) into main (f8932a4) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #269      +/-   ##
==========================================
+ Coverage   81.54%   81.64%   +0.10%     
==========================================
  Files          57       57              
  Lines        5938     5966      +28     
  Branches     1221     1222       +1     
==========================================
+ Hits         4842     4871      +29     
  Misses        869      869              
+ Partials      227      226       -1     

Impacted Files	Coverage Δ
python/lsst/pipe/base/butlerQuantumContext.py	80.00% <100.00%> (+1.50%)	⬆️
tests/test_pipelineTask.py	97.07% <100.00%> (+0.52%)	⬆️

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[timj]

Looks great.