Support deserializing null value in enum array for nullable multi-types

[vegardgs-ksat]

According to the referenced JSON Schema Validation for OAS3.1, the enum property can be of any valid type (presumably matching the multi-valued type fields, including null.

This was observed in the wild when attempting to parse GitHub OpenAPI 3.1 specification, which includes schemas with multi-type, where one type is null.

This can be demonstrated with the following:

use oas3::spec::ObjectSchema;

fn main() {
    let json = serde_json::json!({
        "type": ["string", "null"],
        "enum": ["one", null]
    });

    let object: ObjectSchema = serde_json::from_value(json).unwrap();
}

produces

called `Result::unwrap()` on an `Err` value: Error("invalid type: null, expected a string", line: 0, column: 0)`

The same is true when done through the ObjectOrReference<ObjectSchema>, but with a more diffuse error message:

use oas3::spec::{ObjectOrReference, ObjectSchema};

fn main() {
    let json = serde_json::json!({
        "type": ["string", "null"],
        "enum": ["one", null]
    });

    let reference: ObjectOrReference<ObjectSchema> = serde_json::from_value(json).unwrap();
}

produces

called `Result::unwrap()` on an `Err` value: Error("data did not match any variant of untagged enum ObjectOrReference", line: 0, column: 0)