Default null and blank kwargs are inconsistent with default null and blank kwargs for Django fields about djorm-pgarray

[niwinz]

Basically for storage efficiency. I do not find any relevant reasons to be "consistent" with django defaults.

Also, django warns about storing null for string fields, but says nothing about others types.
Also, changing it, will break backward compatibility.

[mfogel]

Not matching the django defaults here requires adding these fields to the south introspection rules. Manually setting null=False has no affect.

I'll open a pull request with fixes.