Explicitly allow null into CompletableResultCode.failExceptionally()

[breedx-splk]

Relates to this comment: open-telemetry/opentelemetry-android#709 (comment)

This is useful when chaining result codes and wanting to not lose an existing throwable. It was completely valid before, but it wasn't documented.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.10%. Comparing base (1fcbed8) to head (8863f2a).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #6963   +/-   ##
=========================================
  Coverage     90.10%   90.10%           
  Complexity     6601     6601           
=========================================
  Files           730      730           
  Lines         19843    19843           
  Branches       1955     1955           
=========================================
  Hits          17879    17879           
- Misses         1369     1370    +1     
+ Partials        595      594    -1     

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