fix: Unsampled Transactions no longer propagate empty trace headers

[Flash0ver]

This is a follow-up to

• fix: Unsampled spans no longer propagate empty trace headers #4302

#4302 applied the fix to Sentry.AspNetCore only.

Changes

This changeset is

• porting the fix to Sentry.AspNet as well
• adding an additional Test-Case to Sentry.OpenTelemetry/Sentry.OpenTelemetry.Exporter for existing related behavior that is intentionally left unchanged

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.48%. Comparing base (2f2842f) to head (eb9d746).
⚠️ Report is 139 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #5293 +/- ##
==========================================
- Coverage 74.13% 73.48% -0.65% 
==========================================
Files 508 494 -14 Lines 18320 17913 -407 Branches 3584 3494 -90 ==========================================
- Hits 13581 13164 -417 - Misses 3864 3885 +21 + Partials 875 864 -11 

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[Flash0ver]

question: is this a valid test case?

This test fails on Windows on net48.
Is this a valid test case to add (no changes in related production code)?

[jamescrosswell]

Would need to do some digging to confirm, but my gut tells me no.... I don't think we propagate empty trace ids (I seem to recall these get filtered or handled specially when parsing the incoming trace header).