fix(enum): handle nullable enum when useEnums=true

[Kui-kui]

Problem

When OpenAPI specifications combine both enum: [...] and nullable: true, the TypeScript codegen ignores the latter when we pass the useEnums: true param to use enums instead of string unions (example below).

• Nullable values were originally handled in fix: Support for nullable enums #146
• But the fix was not ported when the useEnums argument was implemented in feat: enum support #153

Solution

• I applied the aforementioned fix to the the useEnums early return.
• I also added a new nullable Pet property for the unit tests

Example

The Cat schema is defined as such:

Cat: {
  description: "A cat, meow.",
  type: "object",
  properties: {
    type: {
      type: "string",
    },
    breed: {
      type: "string",
      enum: ["labrador", "carlin", "beagle"],
    },
    temperament: {
      type: "string",
      enum: ["calm", "playful", "aggressive", "shy"],
      nullable: true,
    },
  },
  required: ["type", "breed", "temperament"],
}

Before

With strings ✅

/**
 * A cat, meow.
 */
export type Cat = {
  type: string;
  breed: "labrador" | "carlin" | "beagle";
  temperament: "calm" | "playful" | "aggressive" | "shy" | null;
};

With enums ❌

export enum CatBreed {
  Labrador = "labrador",
  Carlin = "carlin",
  Beagle = "beagle",
}

export enum CatTemperament {
  Calm = "calm",
  Playful = "playful",
  Aggressive = "aggressive",
  Shy = "shy",
}

/**
 * A cat, meow.
 */
export type Cat = {
  type: string;
  breed: CatBreed;
  temperament: CatTemperament;
};

After

With strings (no changes) ✅

/**
 * A cat, meow.
 */
export type Cat = {
  type: string;
  breed: "labrador" | "carlin" | "beagle";
  temperament: "calm" | "playful" | "aggressive" | "shy" | null;
};

With enums ✅

export enum CatBreed {
  Labrador = "labrador",
  Carlin = "carlin",
  Beagle = "beagle",
}

export enum CatTemperament {
  Calm = "calm",
  Playful = "playful",
  Aggressive = "aggressive",
  Shy = "shy",
}

/**
 * A cat, meow.
 */
export type Cat = {
  type: string;
  breed: CatBreed;
  temperament: CatTemperament | null;
};