[BEAM-8473] Fix generator send_type / return_type warning

[alexkoay]

This fixes the generator warning when None is used -- it was checking for None, while the typehint actually returns NoneType.

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ValidatesRunner compliance status (on master branch)

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Java
Python	---			---		---
XLang				---		---

Examples testing status on various runners

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Java	---		---	---	---	---	---
Python	---	---	---	---	---	---	---
XLang	---	---	---	---	---	---	---

Post-Commit SDK/Transform Integration Tests Status (on master branch)

Go	Java	Python

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website	Whitespace	Typescript
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Portable	---			---	---	---

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[alexkoay]

R: @aaltay @charlesccychen

[codecov]

Codecov Report

Merging #15061 (a232b9d) into master (8b66f37) will increase coverage by 0.46%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #15061      +/-   ##
==========================================
+ Coverage   83.76%   84.23%   +0.46%     
==========================================
  Files         439      441       +2     
  Lines       59132    61480    +2348     
==========================================
+ Hits        49534    51786    +2252     
- Misses       9598     9694      +96     

Impacted Files	Coverage Δ
sdks/python/apache_beam/typehints/typehints.py	93.34% <100.00%> (ø)
...on/apache_beam/runners/portability/spark_runner.py	64.81% <0.00%> (-4.76%)	⬇️
...ks/python/apache_beam/runners/worker/data_plane.py	91.21% <0.00%> (-1.52%)	⬇️
sdks/python/apache_beam/runners/direct/executor.py	96.25% <0.00%> (-0.54%)	⬇️
setup.py	0.00% <0.00%> (ø)
sdks/python/apache_beam/dataframe/__init__.py	100.00% <0.00%> (ø)
...ks/python/apache_beam/ml/gcp/recommendations_ai.py	87.26% <0.00%> (ø)
...n/apache_beam/ml/gcp/recommendations_ai_test_it.py	69.76% <0.00%> (ø)
...hon/apache_beam/runners/worker/bundle_processor.py	93.98% <0.00%> (+0.03%)	⬆️
sdks/python/apache_beam/coders/coders.py	88.02% <0.00%> (+0.14%)	⬆️
... and 10 more

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