remove redundant ?? string.Empty in GetSampler

[SimonCropp]

Fixes #
Design discussion issue #

Changes

cant be null since inside a

if (configuration.TryGetStringValue(TracesSamplerConfigKey, out var configValue))

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[SimonCropp]

relies on #6090 being merged first

[github-actions]

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[TimothyMothra]

relies on #6090 being merged first

The other PR merged today. I think we're good to merge this. :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.37%. Comparing base (41e1a59) to head (0c290c0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6078   +/-   ##
=======================================
  Coverage   86.36%   86.37%           
=======================================
  Files         257      257           
  Lines       11579    11578    -1     
=======================================
  Hits        10000    10000           
+ Misses       1579     1578    -1     

Files with missing lines	Coverage Δ
src/OpenTelemetry/Trace/TracerProviderSdk.cs	100.00% <ø> (+0.34%)	⬆️