Return null from TracerShim extract when the carrier has no span context

[thswlsqls]

Fixes #4344

Description

• Propagation.extractTextMap used Context.current() as the extract parent. A TextMapPropagator returns the input context unchanged when the carrier has no valid headers, so an empty-carrier extract with an active span returned that span as a non-null SpanContextShim.
• This violates the OpenTracing Tracer#extract contract: with no serialized span state in the carrier, the result must be null.
• Switching the parent to Context.root() makes extraction depend only on the carrier. A populated carrier decodes exactly as before; inject is left unchanged (preserving the current context there is intended).
• The shim is deprecated but still published, so fixing this contract violation is still worthwhile.

Testing done

• Added TracerShimTest#extract_emptyCarrier_withActiveSpan_returnsNull (fails before the change, passes after).
• ./gradlew :opentracing-shim:check — 83 tests passed.
• Added a ## Unreleased ### Shims entry in CHANGELOG.md. No public API change (apidiff unaffected; Propagation/TracerShim are package-private).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.61%. Comparing base (c5c9d87) to head (fea6e51).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8505      +/-   ##
============================================
+ Coverage     91.60%   91.61%   +0.01%     
- Complexity    10312    10314       +2     
============================================
  Files          1013     1013              
  Lines         27274    27274              
  Branches       3202     3202              
============================================
+ Hits          24983    24986       +3     
+ Misses         1563     1562       -1     
+ Partials        728      726       -2     

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[github-actions]

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