[sdk-traces] SamplingResult constructors accept null parameters

[Kielek]

Fixes open-telemetry/opentelemetry-dotnet-contrib#1718 (comment)
Design discussion issue #

Changes

SamplingResult constructor accepts null parameters

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)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.66%. Comparing base (6250307) to head (3136a49).
Report is 226 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5614      +/-   ##
==========================================
+ Coverage   83.38%   85.66%   +2.28%     
==========================================
  Files         297      254      -43     
  Lines       12531    10982    -1549     
==========================================
- Hits        10449     9408    -1041     
+ Misses       2082     1574     -508     

Flag	Coverage Δ
unittests	?
unittests-Solution-Experimental	85.66% <ø> (?)
unittests-Solution-Stable	85.60% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/OpenTelemetry/Trace/SamplingResult.cs	100.00% <ø> (ø)

... and 114 files with indirect coverage changes