Retain empty tracer name as is instead of using default name

[takaebato]

Fixes #2372

Changes

Ensures empty tracer names are retained, similar to #2334 and #2316.

After implementing this change, I realized the spec might not mention empty tracer names..
Advice on whether this change is necessary would be appreciated!

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: cijothomas / name: Cijo Thomas (5463b57)
• ✅ login: takaebato / name: Takahiro Ebato (ea298ec, e5a7165)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.9%. Comparing base (56e7fae) to head (5463b57).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2486   +/-   ##
=====================================
  Coverage   77.8%   77.9%           
=====================================
  Files        123     123           
  Lines      22887   22925   +38     
=====================================
+ Hits       17824   17870   +46     
+ Misses      5063    5055    -8     

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[cijothomas]

After implementing this change, I realized the spec might not mention empty tracer names..

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#get-a-tracer