fix: no crash when reading the old cache

[frostming]

Fixes #1981

Signed-off-by: Frost Ming me@frostming.com

Pull Request Checklist

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

Describe what you have changed in this PR.

[codecov-commenter]

Codecov Report

Patch coverage: 85.71% and project coverage change: -0.01 ⚠️

Comparison is base (455640a) 85.22% compared to head (b40c58e) 85.22%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1986      +/-   ##
==========================================
- Coverage   85.22%   85.22%   -0.01%     
==========================================
  Files          99       99              
  Lines        9341     9352      +11     
  Branches     2037     2038       +1     
==========================================
+ Hits         7961     7970       +9     
- Misses        937      938       +1     
- Partials      443      444       +1     

Flag	Coverage Δ
unittests	85.01% <85.71%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pdm/models/session.py	94.11% <85.71%> (-5.89%)	⬇️

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