stream: preserve falsy cancellation reasons

[trivikr]

Fixes: #64704

Previously, falsy reasons such as 0, '', false, and null were stored
but later treated as if no error existed. Consumers that had not started
waiting would therefore observe clean completion instead of the supplied
cancellation reason. Sync share also checked whether a consumer was detached
before surfacing its stored cancellation error.

This change uses undefined as the no-error sentinel and checks cancellation
reasons explicitly. Sync share now surfaces the reason before handling the
detached state.

---

Assisted-by: codex:gpt-5.6-sol

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

❌ Patch coverage is 85.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.32%. Comparing base (bfa3e98) to head (0f06c0f).
⚠️ Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/streams/iter/share.js	85.71%	2 Missing ⚠️
lib/internal/streams/iter/broadcast.js	83.33%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #64705   +/-   ##
=======================================
  Coverage   90.32%   90.32%           
=======================================
  Files         760      760           
  Lines      249130   249134    +4     
  Branches    47041    47050    +9     
=======================================
+ Hits       225030   225039    +9     
+ Misses      15490    15487    -3     
+ Partials     8610     8608    -2     

Files with missing lines	Coverage Δ
lib/internal/streams/iter/broadcast.js	87.81% <83.33%> (+0.02%)	⬆️
lib/internal/streams/iter/share.js	85.47% <85.71%> (+0.16%)	⬆️

... and 24 files with indirect coverage changes

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