I have a main Zod schema defined as follows:

const QuestionSchema = z.object({
  Question: z.string(),
  QuestionLevel: z.nativeEnum(QuestionLevel),
  Metadata: z.object({
    Curriculum: z.object({ Name: z.string(), Id: z.number() }),
    Qualification: z.array(z.object({ Name: z.string(), Id: z.number() })),
    Subjects: z.array(z.object({ Name: z.string(), Id: z.number() })).nonempty(),
    Modules: z.array(z.object({ Name: z.string(), Id: z.number() })).nonempty(),
    PastPaperReference: z
      .object({
        Papers: z.array(z.string()).optional(),
        Months: z.array(z.string()).optional(),
        Years: z.array(z.string()).optional(),
      })
      .optional()
      .nullable(),
  }),
});

I then extend this schema to create another schema called SubPartQuestionZodSchema:

const SubPartQuestionZodSchema = QuestionSchema.extend({
  Type: z.literal(QuestionTypes.SUB_PART),
  Questions: z.array(z.lazy(() => QuestionZodSchema)),
  Metadata: z.object({
    ...QuestionSchema.shape.Metadata.shape,
    Modules: z.null().or(z.undefined()),
    Topics: z.null().or(z.undefined()),
    Difficulty: z.null().or(z.undefined()),
    DepthOfKnowledge: z.null().or(z.undefined()),
  }),
});

Finally, I use SubPartQuestionZodSchema in a discriminated union like this:

export const QuestionZodSchema = z.discriminatedUnion("Type", [
  MultiChoiceQuestionZodSchema,
  FillInTheBlankQuestionZodSchema,
  EssayQuestionZodSchema,
  SinglelineZodSchema,
  MultilineZodSchema,
  SubPartQuestionZodSchema,
]);

I need to add a conditional validation to SubPartQuestionZodSchema such that if QuestionLevel === "SUBJECT", then it’s okay for the Modules array to be empty or null. However, if QuestionLevel is any other value, the Modules array should be non-empty and validated according to the original schema.
How can I achieve this conditional validation for the Modules array in the SubPartQuestionZodSchema?