Following @sudip-parajuli's suggestions I modified my PromoCode serializer as below:

class PromoCodeSerializer(serializers.ModelSerializer):
    code = serializers.CharField(allow_blank=True)
    class Meta:
        model = PromoCode

I also overrode the validate method of the OrderSerializer to remove promo_code data when the code value is empty:

def validate(self, data: dict):
    promo_code_data: dict = data.get("promo_code", None)
    if promo_code_data and promo_code_data.get("code"):
        return data
    else:
        data.pop("promo_code", None)
        return data

Solved my problem.