Fix typing of `revision` argument in "Don't Generate Empty Migrations with Autogenera...

Fix typing of `revision` argument in "Don't Generate Empty Migrations with Autogenerate" recipe

### Description

Type-annotations were added to the "Don't Generate Empty Migrations with Autogenerate" cookbook recipe in commit 3e48ed0.  Later, in commit 222e4ba, the type of the `revision` argument to `process_revision_directives()` was changed from `tuple[str, str]` to `str | Iterable[str | None] | Iterable[str]`, but the recipe was not updated.  This PR updates the recipe so that it will type-check.

### Checklist
This pull request is:

- [x] A documentation / typographical 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.

Closes: #1340
Pull-request: https://github.com/sqlalchemy/alembic/pull/1340
Pull-request-sha: 254b4cbe06befaa61a7e11fef5020766f129c7a5

Change-Id: I1697c1d8a3965dae8e696c9f33c179d9f20ef09c