Add some form of support for nullable fields

[yrd]

Currently, an upstream null resolves into this error:

zucker/zucker/model/fields/base.py

Lines 208 to 213 in b45324d

	if raw_value is None:
	raise AttributeError(
	f"Trying to access an undefined field {self.name!r} in record "
	f"{type(record)!r}. Either add the field to the module "
	f"definition or check for the correct spelling."
	)

The first idea would be to add an argument when building the field:

optional_string = model.StringField(optional=True)

Or use a wrapper:

optional_integer = model.OptionalField(model.IntegerField())