Fix NPE when baseline contains empty suppressions

[rubensousa]

Fixes: #79

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.21%. Comparing base (eeebd78) to head (229a925).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
+ Coverage   83.14%   83.21%   +0.07%     
==========================================
  Files          45       45              
  Lines         694      697       +3     
  Branches       52       53       +1     
==========================================
+ Hits          577      580       +3     
  Misses         92       92              
  Partials       25       25              

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