[Exporter.Geneva] stress nullable

[Kielek]

Towards #894

Changes

[Exporter.Geneva] stress tests nullable
With fully nullable repository, it will be easier to accomplish #2251. Unneeded #nullable enable are treated as warnings -> errors in .NET9.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• [ ] Appropriate CHANGELOG.md files updated for non-trivial changes
• [ ] Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.84%. Comparing base (71655ce) to head (60143f3).
Report is 553 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2255       +/-   ##
===========================================
- Coverage   73.91%   62.84%   -11.08%     
===========================================
  Files         267       35      -232     
  Lines        9615     3523     -6092     
===========================================
- Hits         7107     2214     -4893     
+ Misses       2508     1309     -1199     

Flag	Coverage Δ
unittests-Exporter.Geneva	62.84% <ø> (?)

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see 250 files with indirect coverage changes

[CodeBlanch]

LGTM