Support nullable fields about dataclasses-jsonschema

[HHK1]

@s-knibbs

I've encountered an issue with the Nullable type.

What I would like to achieve is to allow Null values to be passed during instantiation with from_dict, but that they remain filtered out.

@dataclass
class WithOptional(JSONSchemaMixin):
    my_field: Optional[str]


WithOptional.from_dict({ "my_field": None }) # Error, None is not of type str

@dataclass
class WithNullable(JSONSchemaMixin):
    my_field: Nullable[str]

WithNullable.from_dict({ "my_field": None }) # Works
WithNullable.from_dict({ "my_field": None }).to_dict() # Includes { "my_field": None }, which is not what I want either

Is there a way to achieve this somehow ? I understand the need for Nullable, but I don't get why optional would fail to init with an explicit null value.