`FileField(null=True)` doesn't work as expected

[Tai Lee]

Given this model:

class Foo(models.Model):
    foo_file = models.FileField(null=True)

which allows null values, if I create a foo object, the foo_file field appears to be None by default, as I would expect. But on save, it is converted to an empty string:

>>> f = Foo()

>>> f.foo_file.name == ''
False

>>> f.foo_file.name == None
True

>>> f.save()
>>> f = Foo.objects.get()

>>> f.foo_file == ''
True

This is both unexpected, and also makes null=True a bit pointless for FileField.

If it is really by design that file fields cannot be null, setting null=True on the field should be an error and the field should have default='' by default. Although I think it would be better for FileField(null=True) to work more like other fields.

[Claude Paroz]

Duplicate of #10244