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 78.77%. Comparing base (824334c) to head (2c22bed).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8505   +/-   ##
=========================================
  Coverage     78.77%   78.77%           
  Complexity     8579     8579           
=========================================
  Files          1009     1009           
  Lines         28993    28993           
  Branches       3599     3599           
=========================================
  Hits          22839    22839           
  Misses         5311     5311           
  Partials        843      843           

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