making a foreignkey not required - django

Question

I have these 2 models:
# core/models.py
Class Certificate:
user = models.Foreignkey(settings.AUTH_USER_MODEL, on_delete=models.SET_NULL,
related_name='certificates')
name = models.Charfield()
.
.
# users/models.py
Class User(AbstractBaseUser):
.
.
and I want to send the certificates of a user along with its other pieces of information.
this is my serializer:
# users/serializers.py
class CertificationSerializer(serializers.ModelSerializer):
"""Serializer for the certificate object"""
class Meta:
model = Certificate
class UserSerializer(serializers.ModelSerializer):
"""Serializer for the user object"""
# TODO
# videos = serializers.PrimaryKeyRelatedField(
# many=True,
# queryset=Tag.objects.all()
# )
certificates = CertificationSerializer(many=True)
class Meta:
model = get_user_model()
# TODO add certificates and videos
fields = ('id', 'email', 'password', 'first_name', 'last_name',
'phone_number', 'credit', 'points', 'certificates')
extra_kwargs = {'password': {'write_only': True,
'label': 'گذرواژه', 'min_length': 5}}
read_only_fields = ('id', 'certificates',)
def create(self, validated_data):
"""Create a new user and return it"""
return get_user_model().objects.create_user(**validated_data)
def update(self, instance, validated_data):
"""Update a user and return it"""
password = validated_data.pop('password', None)
user = super().update(instance, validated_data)
if password:
user.set_password(password)
user.save()
return user
now when I want to create a new user it tells me that field certificates can not be empty, how can I make the certificates field not required?