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.