disallow empty string for baggage name

[brettmc]

spec 1.36.0 clarifies that empty string is not a valid baggage name

see open-telemetry/opentelemetry-specification#4144

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 70.76%. Comparing base (7eb3abb) to head (51f9ec1).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/API/Baggage/BaggageBuilder.php	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1484      +/-   ##
============================================
+ Coverage     70.71%   70.76%   +0.05%     
- Complexity     2761     2762       +1     
============================================
  Files           408      408              
  Lines          8341     8343       +2     
============================================
+ Hits           5898     5904       +6     
+ Misses         2443     2439       -4     

Flag	Coverage Δ
8.1	70.43% <0.00%> (-0.01%)	⬇️
8.2	70.55% <0.00%> (-0.02%)	⬇️
8.3	70.67% <0.00%> (+0.03%)	⬆️
8.4	70.69% <0.00%> (+0.10%)	⬆️
8.5	70.57% <0.00%> (-0.09%)	⬇️

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Files with missing lines	Coverage Δ
src/API/Baggage/BaggageBuilder.php	0.00% <0.00%> (ø)

... and 5 files with indirect coverage changes

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