Remove null=True on CharFields and others

Reviewed CharFields, EmailFields, TextFields, etc.

Some fields were left unaltered:
* proxy_class rely on unique=True, and that only works sensibly with
  null=True
* The same is true for Product's UPC
* Because order's Line model UPC field usually gets fed the Product's
  UPC, it's least intrusive to still allow null=True there
Subtle changes to handle '' instead of None

Tests failed on a few occasions, because CharFields were set to None or
expected to be None.