fix: Empty source tables causing errors when runing pdm

[frostming]

Fixes #2034

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: 100.00% and no project coverage change.

Comparison is base (15b7e88) 84.96% compared to head (143bf13) 84.96%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2043   +/-   ##
=======================================
  Coverage   84.96%   84.96%           
=======================================
  Files          99       99           
  Lines        9419     9419           
  Branches     2060     2060           
=======================================
  Hits         8003     8003           
  Misses        966      966           
  Partials      450      450           

Flag	Coverage Δ
unittests	84.76% <100.00%> (ø)

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

Impacted Files	Coverage Δ
src/pdm/project/config.py	89.09% <100.00%> (ø)
src/pdm/project/core.py	91.08% <100.00%> (ø)

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