Simplified QuerySet field.null handling

QuerySet had previously some complex logic for dealing with nullable
fields in negated add_filter() calls. It seems the logic is leftover
from a time where the WhereNode wasn't as intelligent in handling
field__in=[] conditions.

Thanks to aaugustin for comments on the patch.

tests/regressiontests/queries/models.py

@@ -346,3 +346,9 @@ class OneToOneCategory(models.Model):
 
     def __unicode__(self):
         return "one2one " + self.new_name
+
+class NullableName(models.Model):
+    name = models.CharField(max_length=20, null=True)
+
+    class Meta:
+        ordering = ['id']