Fix: Handle falsy default values in Zod schema generation

Problem

When a schema has default: false (or other falsy values like 0, '', or null), the generated Zod schema incorrectly omits the .default() method and uses .optional() instead. This occurs because the condition if (!required && schema.default) treats falsy values as falsy in JavaScript.

Example from issue #2222:

schema:
  type: object
  properties:
    primary:
      type: boolean
      default: false

Before:

export const patchV2OrganisationsOrganisationIdSchemesSchemeIdPrimaryBody = zod.object({
  "primary": zod.boolean().optional()  // ❌ Missing default
})

Expected:

export const patchV2OrganisationsOrganisationIdSchemesSchemeIdPrimaryBody = zod.object({
  "primary": zod.boolean().default(patchV2OrganisationsOrganisationIdSchemesSchemeIdPrimaryBodyPrimaryDefault)
})

Solution

Changed the condition from schema.default to schema.default !== undefined to properly detect when a default value is set, regardless of whether it's a falsy value.

// Changed from:
if (!required && schema.default) {

// To:
if (!required && schema.default !== undefined) {

This ensures that all default values, including falsy ones, are properly applied to the Zod schema.