Fix shell detection failure

[pgarrison]

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.

Now, if pdm venv activate fails to identify the shell (as it did with my zsh setup) it will use the default behavior instead of crashing.

Fixes #2187

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (79a56a7) 84.27% compared to head (1d42c4c) 84.27%.
Report is 1 commits behind head on main.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2188   +/-   ##
=======================================
  Coverage   84.27%   84.27%           
=======================================
  Files         100      100           
  Lines        9786     9786           
  Branches     2161     2161           
=======================================
  Hits         8247     8247           
  Misses       1065     1065           
  Partials      474      474           

Flag	Coverage Δ
unittests	84.04% <ø> (ø)

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Files Changed	Coverage Δ
src/pdm/cli/commands/venv/activate.py	92.30% <ø> (ø)

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