Do not overwrite existing baggage with empty baggage in JaegerPropagator

[thswlsqls]

Fixes #8630

Description

• getBaggageFromHeader() built a Baggage.builder() for any non-null jaeger-baggage header value, so a header with no parsable key=value entry returned an empty but non-null Baggage. extract() stored it, and Baggage.storeInContext() overwrites unconditionally: under otel.propagators=tracecontext,baggage,jaeger, W3C baggage was silently dropped.
• Return null when the built baggage is empty; the method is already @Nullable and the caller checks for null.
• This matches three siblings that already guard it: the uberctx- branch of the same method, OtTracePropagator.extract(), and W3CBaggagePropagator.extractMulti(). Apply baggage limits #8380 added those but left this branch.
• Well-formed headers, byte accounting, and limits are unchanged.

Testing done

• Added JaegerPropagatorTest#extract_baggageOnly_withHeader_invalid_keepsExistingBaggage, a @ParameterizedTest over nometa+novalue, user=, an empty header, and a=b=c, each starting from a context holding baggage.
• Not vacuous: reverting the fix fails all 4 cases.
• ./gradlew :extensions:trace-propagators:check --rerun-tasks — 135 tests passed.
• No apidiff (private method body). Added a ## Unreleased CHANGELOG.md entry.
• *IT and GraalVM native-image tests were not run locally.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.61%. Comparing base (b86f040) to head (eed0c49).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8632   +/-   ##
=========================================
  Coverage     91.60%   91.61%           
- Complexity    10343    10344    +1     
=========================================
  Files          1013     1013           
  Lines         27352    27355    +3     
  Branches       3215     3216    +1     
=========================================
+ Hits          25057    25060    +3     
  Misses         1567     1567           
  Partials        728      728           

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