fix `server_default` type hint

fix `server_default` type hint

### Description

The docstring of `alter_column` states: "Set to `None` to have the default removed". The type hint should accept None.

### Checklist

This pull request is:

- [ ] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [x] 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.

Fixes: #1639
Closes: #1641
Pull-request: https://github.com/sqlalchemy/alembic/pull/1641
Pull-request-sha: 6642187d254681712443039a95cc1f3110549654

Change-Id: Icb7246b47805d9b2c1ab70d4e26f3e09354fe190