stream: skip null output from stateful transforms

[trivikr]

Fixes: #64461

Stateful transforms currently pass null output to the flattening
helpers, causing ERR_INVALID_ARG_TYPE. Transform output
normalization defines null as no output.

Skip null before flattening stateful transform output in both the
sync and async paths.

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74768/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.24%. Comparing base (14166a5) to head (3c2ae5a).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64462      +/-   ##
==========================================
+ Coverage   90.23%   90.24%   +0.01%     
==========================================
  Files         741      741              
  Lines      241375   241606     +231     
  Branches    45483    45536      +53     
==========================================
+ Hits       217800   218036     +236     
+ Misses      15129    15109      -20     
- Partials     8446     8461      +15     

Files with missing lines	Coverage Δ
lib/internal/streams/iter/pull.js	83.89% <100.00%> (+0.03%)	⬆️

... and 35 files with indirect coverage changes

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