Use empty slices instead of nil

[zhengkezhou1]

Which problem is this PR solving?

• Resolves Improve: Translator use empty struct instead of nil pointer #6798

Description of the changes

• Added boundary case handling for empty fields during model conversion.

How was this change tested?

• unit tests

Checklist

• I have read https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
• I have signed all commits
• I have added unit tests for the new functionality
• I have run lint and test steps successfully
  • for jaeger: make lint test
  • for jaeger-ui: npm run lint and npm run test

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.03%. Comparing base (300cc7c) to head (98037cd).
Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6799      +/-   ##
==========================================
- Coverage   96.04%   96.03%   -0.01%     
==========================================
  Files         363      364       +1     
  Lines       20677    20700      +23     
==========================================
+ Hits        19859    19880      +21     
- Misses        624      626       +2     
  Partials      194      194              

Flag	Coverage Δ
badger_v1	9.90% <100.00%> (+0.14%)	⬆️
badger_v2	1.98% <100.00%> (+0.16%)	⬆️
cassandra-4.x-v1-manual	14.94% <100.00%> (+0.13%)	⬆️
cassandra-4.x-v2-auto	1.97% <100.00%> (+0.16%)	⬆️
cassandra-4.x-v2-manual	1.97% <100.00%> (+0.16%)	⬆️
cassandra-5.x-v1-manual	14.94% <100.00%> (+0.13%)	⬆️
cassandra-5.x-v2-auto	1.97% <100.00%> (+0.16%)	⬆️
cassandra-5.x-v2-manual	1.97% <100.00%> (+0.16%)	⬆️
elasticsearch-6.x-v1	19.27% <100.00%> (+0.12%)	⬆️
elasticsearch-7.x-v1	19.35% <100.00%> (+0.12%)	⬆️
elasticsearch-8.x-v1	19.52% <100.00%> (+0.12%)	⬆️
elasticsearch-8.x-v2	1.98% <100.00%> (+0.16%)	⬆️
grpc_v1	10.95% <100.00%> (+0.14%)	⬆️
grpc_v2	7.95% <100.00%> (+0.15%)	⬆️
kafka-3.x-v1	10.20% <100.00%> (+0.07%)	⬆️
kafka-3.x-v2	1.98% <100.00%> (+0.16%)	⬆️
memory_v2	1.98% <100.00%> (+0.16%)	⬆️
opensearch-1.x-v1	19.40% <100.00%> (+0.12%)	⬆️
opensearch-2.x-v1	19.40% <100.00%> (+0.12%)	⬆️
opensearch-2.x-v2	1.98% <100.00%> (+0.16%)	⬆️
tailsampling-processor	0.48% <0.00%> (-0.01%)	⬇️
unittests	94.92% <100.00%> (-0.01%)	⬇️

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