Handle objects without MD5 digests

[jwodder]

Closes #13.

[codecov]

Codecov Report

Attention: Patch coverage is 3.70370% with 26 lines in your changes missing coverage. Please review.

Project coverage is 40.36%. Comparing base (ee41195) to head (3b2d603).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/inventory.rs	6.66%	14 Missing ⚠️
src/s3/mod.rs	0.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
- Coverage   41.50%   40.36%   -1.15%     
==========================================
  Files          10       10              
  Lines         530      545      +15     
==========================================
  Hits          220      220              
- Misses        310      325      +15     

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