Fix OTLP marshaling for empty string attributes

[trask]

Based on https://protobuf.dev/programming-guides/proto3/#oneof

If you set a oneof field to the default value (such as setting an int32 oneof field to 0), the “case” of that oneof field will be set, and the value will be serialized on the wire.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.24%. Comparing base (6893380) to head (614e662).

Files with missing lines	Patch %	Lines
...orter/internal/marshal/StatelessMarshalerUtil.java	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8014      +/-   ##
============================================
+ Coverage     90.23%   90.24%   +0.01%     
- Complexity     7602     7604       +2     
============================================
  Files           838      838              
  Lines         22806    22807       +1     
  Branches       2274     2272       -2     
============================================
+ Hits          20578    20583       +5     
+ Misses         1518     1516       -2     
+ Partials        710      708       -2     

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