trace: SpanFromContext and SpanContextFromContext make no allocs

[pellared]

Without this SpanFromContext and SpanContextFromContext produce an allocation when the provided context has no span (e.g. nil, context.Background).

Discovered when working on: #4955

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.4%. Comparing base (9184b10) to head (f27ab1c).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5049   +/-   ##
=====================================
  Coverage   83.4%   83.4%           
=====================================
  Files        238     238           
  Lines      15745   15745           
=====================================
  Hits       13145   13145           
  Misses      2312    2312           
  Partials     288     288           

Files	Coverage Δ
trace/context.go	100.0% <100.0%> (ø)
trace/noop.go	57.8% <100.0%> (ø)

[XSAM]

👍