[BEAM-8473] Fix minor issue in the Generator type hint

[adesormi]

The Generator type hint should only contain the YieldType. SendType and ReturnType are supposed to be left None and warnings are triggered if this is not the case.
The code was checking twice "send_type" and never "return_type". The condition and the warning message were not in line.
This PR aims at fixing this minor bug.

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Post-Commit Tests Status (on master branch)

Lang	SDK	Dataflow	Flink	Samza	Spark	Twister2
Go		---		---		---
Java
Python				---		---
XLang		---		---		---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

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[udim]

Thanks for this!
I'll update the title with the related bug (https://issues.apache.org/jira/browse/BEAM-8473)

[udim]

Opened bug for PythonDocker failure: https://issues.apache.org/jira/browse/BEAM-10567