Using a ManyToManyWidget does not allow to ignore an empty value

[nemesifier]

Describe the bug

    def clean(self, value, row=None, **kwargs):
        if not value:
            return self.model.objects.none()

Try this in the python shell:

>>> self.model.objects.none() == self.model.objects.none()
False

This means that even modifying a field to add Model.objects.none() to the empty values doesn't work to ignore the empty queryset, so the cleaned value is always saved.

This can cause issues when the model being imported is referenced by an optional OneToOne relationship from another model and this model has an optional m2m, in this case the m2m is saved even when it shouldn't and it raises an error.

To Reproduce
...

Versions (please complete the following information):

• Django Import Export: 3.30
• Python 3.8.10
• Django 4.2

Expected behavior

I think we could return an empty list instead of an empty queryset in this case, that way we can add '[]' to empty_values in Field to solve it. Alternatively we could just return an empty string and modify the rest of the code to ignore falsy or empty values.

Screenshots

[matthewhegarty]

Thanks for raising. I would suggest to look at release 4. We will be changing the interface in a few places to standardize it, so would be a good opportunity to put a fix in. I think that returning an empty list would work ok, so no need to modify Field.

If you felt like creating a PR, that would be appreciated. Contributing guidelines are here.