Mysql db check should handle None db_type returns. Backport from https:/...

When implementing a custom field type with a db_type() method that returns None, on Django 1.6 and below we would signify not to create a column at all.
With Django 1.7, this causes a crash with the mysql backend.
The issue is already fixed in master and this simply backports the safety check.

Backport from e910340#diff-14.

https://code.djangoproject.com/ticket/23761