from rest_framework import serializers

class ModelSerializer(serializers.ModelSerializer):

    def to_internal_value(self, data):
        for field_name, value in data.items():
            if not field_name in self.fields:
                continue
            field_object = self.fields[field_name]
            if all([
                value == "",
                field_object.allow_null,
                any([
                    isinstance(field_object, serializers.DateField),
                    isinstance(field_object, serializers.DateTimeField),
                ]),
            ]):
                data[field_name] = None
        return super().to_internal_value(data)