Fix type annotation in `op.create_table_comment`

Fix type annotation in `op.create_table_comment`

### Description

The `existing_comment` parameter had type `None`, but it should be `Optional[str]`.

Fixes #903

### 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.

**Have a nice day!**

Closes: #1115
Pull-request: https://github.com/sqlalchemy/alembic/pull/1115
Pull-request-sha: 973ca93b199dccf4474db650bcfee797b2bf7c51

Change-Id: Ic4a25e320fe3e4578e3df9aea689d760844c055e