Zod mail configuration schema that takes a default empty value for an email field but also can be an empty string

schema.ts

export const mailConfigurationSchema = z.object({
  apiKey: z.string().default(""),
  fromEmail: z
    .string()
    .email()
    .optional()
    .or(z.literal(""))
    .default(""),
  toName: z.string().default(""),
  toEmail: z
    .string()
    .email()
    .optional()
    .or(z.literal(""))
    .default(""),
});