index: md5: handle missing files and cloud versioned files missing version_id

[efiop]

For the record, there will be a higher level test in dvc, but I'll likely come back to adding more to dvc-data as well.

[codecov-commenter]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (31a65d7) 63.83% compared to head (c7bca3b) 63.70%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/dvc_data/index/index.py	14.28%	5 Missing and 1 partial ⚠️
src/dvc_data/index/save.py	50.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #485      +/-   ##
==========================================
- Coverage   63.83%   63.70%   -0.13%     
==========================================
  Files          61       61              
  Lines        4164     4177      +13     
  Branches      712      714       +2     
==========================================
+ Hits         2658     2661       +3     
- Misses       1358     1368      +10     
  Partials      148      148              

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