Error construct schema for ListAttribute without `of`

[askerka]

Hi!
There is a problem with ListAttribute():

from pynamodb.models import Model
from marshmallow_pynamodb import ModelSchema


class User(Model):
    nick_names = ListAttribute()


class UserSchema(ModelSchema):
    class Meta:
        model = User

/marshmallow_pynamodb/schema.py", line 39, in get_declared_fields
    element_type = type(attribute.element_type.__name__, (ModelSchema, ), {'Meta': Meta})
AttributeError: 'NoneType' object has no attribute '__name__'

It is understandable that is not so much sense for schema with list without type specification but it is valid model and possible use case.

So it seems two way to handle this issue. First option is forbid create list attribute without specifying type and second is handle this situation and create schema and validate only container type.

[askerka]

@mathewmarcus Hi! If you could choose way how to handle this situation I would like to create PR to close issue

[mathewmarcus]

@asduj I would welcome a PR. As far as how to resolve this issue, I would prefer the second approach (i.e. handle the situation where a ListAttribute doesn't have a type)