stream: handle sync writev completion in pipeTo

[trivikr]

Fixes: #63560

Fixes stream/iter pipeTo() when a duck-typed writer accepts a batched
writev() synchronously and returns undefined.

pipeTo() already tolerated undefined from single write() calls, but the
writev() path passed the return value directly to PromisePrototypeThen(),
which caused a TypeError for synchronous completion.

---

Assisted-by: openai:gpt-5.5

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.32%. Comparing base (49dcaa0) to head (825d2c9).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63561      +/-   ##
==========================================
- Coverage   90.33%   90.32%   -0.02%     
==========================================
  Files         730      730              
  Lines      234362   234472     +110     
  Branches    43906    43916      +10     
==========================================
+ Hits       211713   211781      +68     
- Misses      14371    14433      +62     
+ Partials     8278     8258      -20     

Files with missing lines	Coverage Δ
lib/internal/streams/iter/pull.js	88.21% <100.00%> (+0.09%)	⬆️

... and 43 files with indirect coverage changes

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