`AsyncGenerator` should not yield `None`

[DimitriPapadopoulos]

The None was added in 0e035fb / #1844, let's try to remove it.

TODO:

• [ ] Add unit tests and/or doctests in docstrings
• [ ] Add docstrings and API docs for any new/modified user-facing classes and functions
• [ ] New/modified features documented in docs/tutorial.rst
• [ ] Changes documented in docs/release.rst
• [x] GitHub Actions have all passed
• [x] Test coverage is 100% (Codecov passes)

[DimitriPapadopoulos]

Very informative. I'll read it more closely, but so far I understand there are two options:

• the recommended option, no async in the abstract method,
• the alternative, async and phony yield in the abstract method.

In any case, the above applies to #2381. I have decided to remove None in this separate PR, as things were getting pretty complex.