Fix nullability of blank charfields in values and values_list (#1020)

Char fields with blank=True set should not be considered nullable in the
context of values() and values_list() querysets.

I'm also not a huge fan of the way these fields are made optional in the
constructur to the model classes, it feels like it would be better to
mark the arguments as having a default value, rather than allow sending
in None, but I'd rather keep this fix small and look at the overall
problem at a later point.

mypy_django_plugin/django/context.py

@@ -263,6 +263,9 @@ class DjangoContext:
         return attname
 
     def get_field_nullability(self, field: Union[Field, ForeignObjectRel], method: Optional[str]) -> bool:
+        if method in ("values", "values_list"):
+            return field.null
+
         nullable = field.null
         if not nullable and isinstance(field, CharField) and field.blank:
             return True