Fix `get_one` return value description

Fix `get_one` return value description

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### Description
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The `get_one()` method is described as raising an error if no row is found instead of returning `None` (as `get()` does). Also, the return type hint prohibits `None`.

### Checklist
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This pull request is:

- [x] A documentation / typographical / small typing error fix
	- Good to go, no issue or tests are needed
- [ ] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #10473
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/10473
Pull-request-sha: 1cd2c2a2df2c91a303e459c8f0c5dc950d8c7b39

Change-Id: Iaf69b05d99f4ed0df922f34f27be0b46c1410f7e