Handle "nullable" array types

[tal-franji]

If given "nullable":true in schema - array value still fails on null value.
Suggested patch to schemas.py:

class Model:
    """
    A Model is a representation of a Schema as a request or response.  Models
    are generated from Schema objects by called :any:`Schema.model` with the
    contents of a response.
    """

    __slots__ = ["_raw_data", "_schema"]

    def __init__(self, data, schema):
        """
        Creates a new Model from data.  This should never be called directly,
        but instead should be called through :any:`Schema.model` to generate a
        Model from a defined Schema.

        :param data: The data to create this Model with
        :type data: dict
        """
        data = data or {}

        self._raw_data = data
        self._schema = schema

        for s in self.__slots__:
            # initialize all slots to None
            setattr(self, s, None)

        keys = set(data.keys()) - frozenset(self.__slots__)
        if keys:
            raise ModelError("Schema {} got unexpected attribute keys {}".format(self.__class__.__name__, keys))

        # collect the data into this model
        for k, v in data.items():
            prop = schema.properties[k]
            if prop.nullable and v is None:  #  added
                setattr(self, k, None)  # added
            elif prop.type == "array":  # change to elif
                # handle arrays
                item_schema = prop.items
                if v is None:   # added
                    raise ValueError(f"fiels '{k}' of type 'array' cannont be null (unless declared as nullable)")  # added
                setattr(self, k, [item_schema.model(c) for c in v])
            elif prop.type == "object":
                # handle nested objects
                object_schema = prop
                setattr(self, k, object_schema.model(v))
            else:
                setattr(self, k, v)

    def __repr__(self):
        """
        A generic representation of this model
        """
        return str(dict(self))

    def __iter__(self):
        for s in self.__slots__:
            if s.startswith("_"):
                continue
            yield s, getattr(self, s)
        return

[commonism]

Hi,

can you provide an example description document and input data which could be used to create a unit test?