Increase use of NotNullWhen polyfill

[SimonCropp]

Fixes #
Design discussion issue #

Changes

also allows us to remove some redundant ! null suppressions

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)

[TimothyMothra]

LGTM. You have conflicts with main and will need to update your branch

[SimonCropp]

conflicts resolved

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.36%. Comparing base (720d610) to head (0fec244).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6090      +/-   ##
==========================================
- Coverage   86.36%   86.36%   -0.01%     
==========================================
  Files         257      257              
  Lines       11584    11579       -5     
==========================================
- Hits        10005    10000       -5     
  Misses       1579     1579              

Files with missing lines	Coverage Δ
...etryProtocol/Implementation/ExperimentalOptions.cs	88.23% <ø> (-1.24%)	⬇️
...Telemetry.Exporter.Zipkin/ZipkinExporterOptions.cs	100.00% <ø> (ø)
src/OpenTelemetry/Logs/LoggerProviderSdk.cs	92.55% <ø> (ø)
...OpenTelemetry/Resources/OtelEnvResourceDetector.cs	100.00% <ø> (ø)
...lemetry/Resources/OtelServiceNameEnvVarDetector.cs	100.00% <ø> (ø)