[Exporter.OTLP - Tests] Nullable

[Kielek]

Follow up to #5708

Changes

[Exporter.OTLP - Tests] Nullable

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 86.18%. Comparing base (6250307) to head (61a7465).
Report is 271 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5720      +/-   ##
==========================================
+ Coverage   83.38%   86.18%   +2.79%     
==========================================
  Files         297      254      -43     
  Lines       12531    11057    -1474     
==========================================
- Hits        10449     9529     -920     
+ Misses       2082     1528     -554     

Flag	Coverage Δ
unittests	?
unittests-Project-Experimental	86.17% <100.00%> (?)
unittests-Project-Stable	86.11% <100.00%> (?)

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

Files	Coverage Δ
...orter.OpenTelemetryProtocol/OtlpExporterOptions.cs	99.08% <100.00%> (+2.71%)	⬆️

... and 197 files with indirect coverage changes

[CodeBlanch]

LGTM