Fix birthtime if not present

[Schamper]

(DIS-1659)

[codecov]

Codecov Report

Merging #10 (d223a2d) into main (abd3482) will decrease coverage by 0.95%.
The diff coverage is 0.00%.

❗ Current head d223a2d differs from pull request most recent head a7cbfc3. Consider uploading reports for the commit a7cbfc3 to get more accurate results

@@            Coverage Diff             @@
##             main      #10      +/-   ##
==========================================
- Coverage   66.42%   65.46%   -0.96%     
==========================================
  Files           4        4              
  Lines         274      278       +4     
==========================================
  Hits          182      182              
- Misses         92       96       +4     

Flag	Coverage Δ
unittests	65.46% <0.00%> (-0.96%)	⬇️

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Impacted Files	Coverage Δ
dissect/ffs/ffs.py	63.49% <0.00%> (-0.99%)	⬇️

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[yunzheng]

small comment, otherwise LGTM

[yunzheng]

LGTM