[Api] Fix empty tracestate extraction

[Kielek]

Broader fix of open-telemetry/opentelemetry-dotnet-contrib#4503

Changes

Normalize valid empty tracestate header values to null when extracting W3C trace context with TraceContextPropagator.

The W3C Trace Context specification requires vendors to accept empty tracestate headers, while recommending that senders avoid producing them:
https://www.w3.org/TR/trace-context/#tracestate-header-field-values

Before this change, a present-but-empty tracestate header extracted as ActivityContext.TraceState == "", while an absent tracestate header extracted as null. Both represent no trace state, but the observable difference can cause downstream code comparing OpenTelemetry-extracted context with runtime-created Activity context to treat them as different parents.

This change keeps malformed non-empty tracestate behavior unchanged, but normalizes valid empty or whitespace-only tracestate values to null.

Tests

Added coverage for present empty/whitespace tracestate values:

• tracestate: ""
• tracestate: " "
• tracestate: " , "

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 89.71%. Comparing base (b01e1d6) to head (481e87a).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7407      +/-   ##
==========================================
- Coverage   89.72%   89.71%   -0.01%     
==========================================
  Files         276      276              
  Lines       14576    14576              
==========================================
- Hits        13078    13077       -1     
- Misses       1498     1499       +1     

Flag	Coverage Δ
unittests-Project-Experimental	89.49% <100.00%> (-0.15%)	⬇️
unittests-Project-Stable	89.46% <100.00%> (-0.20%)	⬇️

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

Files with missing lines	Coverage Δ
....Api/Context/Propagation/TraceContextPropagator.cs	90.98% <100.00%> (+0.29%)	⬆️

... and 3 files with indirect coverage changes