fix: don't return empty string when trace header is not present

[TommyCpp]

Fixes #1475
Design discussion issue (if applicable) #

Changes

• if trace header is not present. Just return default

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)

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (8688e7e) 61.8% compared to head (1068d8d) 61.9%.
Report is 4 commits behind head on main.

Files	Patch %	Lines
opentelemetry-sdk/src/metrics/periodic_reader.rs	92.3%	4 Missing ⚠️
opentelemetry-sdk/src/logs/log_emitter.rs	90.9%	1 Missing ⚠️
opentelemetry/src/logs/noop.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1523   +/-   ##
=====================================
  Coverage   61.8%   61.9%           
=====================================
  Files        144     144           
  Lines      19810   19862   +52     
=====================================
+ Hits       12256   12303   +47     
- Misses      7554    7559    +5     

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[hdost]

Looks good! are there tests we want to add?