Support x-nullable OpenAPI 2.0 extension

[johnthagen]

Describe the solution you'd like
x-nullable is a very common vendor extension that allows expressing nullabilty in an OpenAPI 2.0 schema. Tools such as swagger-codegen and openapi-generator can use this to preserve this type information into the generated code. This is important when exposing database columns that can be NULL.

The DjangoRestFramework ecosystem has support for this.

Using x-nullable, would change the generated TypeScript from

id: number;

to

id: number | null;

The client code can then use type checking to ensure that null is properly checked before accessing the content.

This could be done by adding an optional nullable keyword argument to fields, for example:

my_model = api.model('MyModel', {
    'id': fields.Integer(nullable=True),
}

[maartenvdmeulen]

Any updates?
I like to use x-nullable with openapi-typescript-codegen. See #Nullable in OpenAPI v2