DecimalField raise exception if min_value is set and input data is empty string

[msscaroso]

Hello,
First of all, thank you for the amazing framework. I have been using it since I started to work as a developer and it really helps me on a day to day basis.
I'm raising a potential issue and, if confirmed that it's actually an issue, I'm more than happy to work on a patch !

Consider the example below:

class WithoutMin(serializers.Serializer):
    bar_number = serializers.DecimalField(
        required=False,
        allow_null=True,
        max_digits=11,
        decimal_places=3,
    )


class WithMin(serializers.Serializer):
    bar_number = serializers.DecimalField(
        required=False, allow_null=True, max_digits=11, decimal_places=3, min_value=0
    )


def test_empty_string():
    serializer = WithoutMin(data={"bar_number": ""})
    # empty string is considered a valid value
    assert serializer.is_valid()

    serializer = WithMin(data={"bar_number": ""})
    # but if min_value is set, this raises an exception
    serializer.is_valid()

As we can see, the first serializer considers the empty string a valid value. Maybe someone can disagree with this behavior, but it's not the point.

But if I declare the same serializer, but adding a min_value=0 constraint in the DecimalField it throws the following exception:

    def compare(self, a, b):
>       return a < b
E       TypeError: '<' not supported between instances of 'NoneType' and 'int'

I believe it should either consider the blank string as invalid or handle it. But throwing an exception should not happen.

Tested this with 3.12.4

[msscaroso]

The line where the exception happens is part of the django project (https://docs.djangoproject.com/en/4.1/_modules/django/core/validators/#MinValueValidator)

But I think the django validator expects this value to be already cleaned.
In fact, rest framework doesn't fail if I send None instead of "".