Fix empty neuron.__version__ when git describe fails in shallow clones

[nrnhines]

Summary

• Release dry-run wheels use a depth-1 checkout and do not create the release tag, so git describe fails while .git still exists.
• add_cpp_git_information left GIT_DESCRIBE empty in that case, so neuron.__version__ (nrnversion(5)) was "" and ModelDB CI failed on runmodels --workdir=$nrn_ver (mkdir('')).
• Fall back to PROJECT_VERSION when describe fails/empty so dry-run and ship wheels get the same non-empty string (e.g. 9.0.2).
• Assert non-empty __version__ in test_wheels.sh.

Test plan

• Wheel CI / NEURON Release dry-run: python -c "import neuron; print(repr(neuron.__version__))" is non-empty (e.g. '9.0.2')
• ModelDB V2 no longer fails with empty workdir after installing dry-run wheels
• Ship path (upload=true, tag checkout) still reports the tag version via describe

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[nrnhines]

@ramcdougal Please approve.
The NEURON release workflow for 9.0.2 has gotten further on each of the sequence of changes to master, release/9.0, and nrn-modeldb-ci and hopefully (always hopefully) this will be the last. But it's no way to run a release process. After this release, we'll merge #3838 to master and thereafter I believe we can do all release workflow tweaking and testing using PR's instead of constantly having to merge fixes to master, release/9.x, and nrn-modeldb-ci.

[azure-pipelines]

✔️ d668f5d -> Azure artifacts URL

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.17%. Comparing base (7ca2dbb) to head (d668f5d).

Additional details and impacted files

@@             Coverage Diff              @@
##           release/9.0    #3840   +/-   ##
============================================
  Coverage        68.17%   68.17%           
============================================
  Files              688      688           
  Lines           111016   111016           
============================================
  Hits             75686    75686           
  Misses           35330    35330           

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