[Instrumentation.AspNetCore] Fix missing grpc.method tag when empty tracestate header is present

[chinmaychahar]

Fixes #4501

Changes

Normalize null and empty string TraceState comparison in HttpInListener.OnStartActivity to prevent sibling activity creation when an empty tracestate: header is present.

The .NET runtime's W3CPropagator sets activity.TraceStateString = null for an empty tracestate header, while OpenTelemetry's CompositeTextMapPropagator extracts TraceState = "". Both represent absent tracestate but== comparison treats them as different, triggering unnecessary sibling activity creation.

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 75.86%. Comparing base (d2cb84e) to head (537ffd7).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4503   +/-   ##
=======================================
  Coverage   75.86%   75.86%           
=======================================
  Files         438      438           
  Lines       18002    18002           
=======================================
  Hits        13657    13657           
  Misses       4345     4345           

Flag	Coverage Δ
unittests-Instrumentation.AspNet	77.50% <ø> (ø)
unittests-Instrumentation.AspNetCore	87.34% <100.00%> (ø)

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

Files with missing lines	Coverage Δ
...tation.AspNetCore/Implementation/HttpInListener.cs	87.07% <100.00%> (ø)

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[Kielek]

I think that it should be handled in this way open-telemetry/opentelemetry-dotnet#7407

[chinmaychahar]

Thanks @Kielek! That's a cleaner fix. Since #7407 is already merged, should I close this PR?

[Kielek]

@chinmaychahar, closing this PR makes sense to me. The issue will be after next OTel SDK release.
We have made it last week, so it takes some time. If the fix is urgent, you can consider reopening here and make this fix local.
Instrumentation packages are released usually on demand.