fix: crash when comparing metrics with None values

[mdqst]

updated compare_subdicts and markdown_subtable to safely handle cases where old[file] or new[file] might be None.
using (old.get(file) or {}).get(metric) and (new.get(file) or {}).get(metric) prevents AttributeError and ensures the comparison works reliably.

PR Checklist

• Tests
• Documentation
• Added entry to CHANGELOG.md
• Tried the feature on a public network

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.38%. Comparing base (fc94206) to head (0c75053).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1529   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files          82       82           
  Lines        2272     2272           
=======================================
  Hits         2099     2099           
  Misses        173      173           

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