to_json should initialize None when doesn't exist in database

[Polandia94]

to_json doen't project null fields. In this case, to_json() have important differences with the model.

class Holding(MapAttribute):
    id_holding= NumberAttribute()
    available = NumberAttribute(null=True)

class HoldingModel(Model):
    id_number = NumberAttribute(hash_key=True)
    holdings = ListAttribute(of=Holding)

# doesnt work
for object in HoldingModel.query(1):
    object.to_json()["holdings"][0]['available']

# work
for object in HoldingModel.query(1):
    object..holdings[0].available
    

As available doesn't exist in dynamodb (because null fields aren't saved), it doesn't exist in my model.

if the available have default=None, in the json the available doesn't exist, if the default is 0, in the json the available exist