Skip empty baggage key in OtTracePropagator extract

[thswlsqls]

Fixes #8629

Description

• OtTracePropagator.extract() only checked startsWith(PREFIX_BAGGAGE_HEADER), so a header named exactly ot-baggage- produced a baggage entry keyed "". inject() re-emits it, and it consumes one of the 64 baggage entries.
• Skip a header whose length equals the prefix length, matching JaegerPropagator.getBaggageFromHeader() and its extract_baggageOnly_withPrefix_emptyKey test in the same package.
• An empty key is not a valid baggage key here: W3CBaggagePropagator.isValidBaggageKey() rejects it, and the ImmutableKeyValuePairs Javadoc makes removing empty keys the caller's responsibility.
• Handling of normal ot-baggage-<key> headers, byte accounting, and limits are unchanged.

Testing done

• Added OtTracePropagatorTest#extract_Baggage_EmptyKey, asserting Baggage.empty() for a valid span context plus an ot-baggage- header.
• Not vacuous: reverting the production change fails it.
• ./gradlew :extensions:trace-propagators:check --rerun-tasks — 132 tests passed.
• No apidiff (method body only) and no CHANGELOG (only malformed entries stop being extracted).
• *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 91.60%. Comparing base (b86f040) to head (8045f71).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8631   +/-   ##
=========================================
  Coverage     91.60%   91.60%           
- Complexity    10343    10344    +1     
=========================================
  Files          1013     1013           
  Lines         27352    27353    +1     
  Branches       3215     3216    +1     
=========================================
+ Hits          25057    25058    +1     
  Misses         1567     1567           
  Partials        728      728           

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