uniqueness validator at the serializer level does not support add default automatically.

[elonzh]

ModelSerializer will add default automatically when generate uniqueness extra kwargs from Model definition(by rest_framework.serializers.ModelSerializer.get_uniqueness_extra_kwargs), which is useful when we have nullable uniqueness constraints.

But when we want to add a uniqueness validator at the serializer level, this feature is disabled and we need to set default explicitly.