Modified [7760] to not include a "col is not NULL" fragment for non-nullable fields.

This avoids any use of "pk is not NULL" fragment, which behave inconsistently
in MySQL. Thanks to Russell Keith-Magee for diagnosing the problem and
suggesting the easy fix.

Refs #7076.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7812 bcc190cf-cafb-0310-a4f2-bffc1f526a37