Stopped using null=True on CharFields

null=True is only useful for a few edge cases where one wants to
separate between '' and None.
If null=True was found, it was removed. If blank=True was not set
already, it was set instead.
If default=None was set, it was removed (defaulting to '').

I then grepped the source code for tests 'is None' and 'is not None',
and checked the instances where default=None was set.