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

django/db/models/sql/query.py

@@ -1131,7 +1131,7 @@ class Query(object):
                             entry.negate()
                             self.where.add(entry, AND)
                             break
-                elif not (lookup_type == 'in' and not value):
+                elif not (lookup_type == 'in' and not value) and field.null:
                     # Leaky abstraction artifact: We have to specifically
                     # exclude the "foo__in=[]" case from this handling, because
                     # it's short-circuited in the Where class.