Optional enum set to undefined serializes as -1 and breaks unpack()

[mirismaili]

Description

When an optional enum field is explicitly set to undefined during pack(), it is serialized as enum index -1. The resulting payload is accepted by pack() but cannot be read back by unpack().

Omitting the same optional property entirely works correctly.

Versions

• gramio: 0.8.3
• @gramio/callback-data: 0.1.0

Minimal reproduction

import {CallbackData} from '@gramio/callback-data'

const action = new CallbackData('example-action')
  .string('itemId')
  .enum('deliveryMethod', ['email', 'sms'], {optional: true})

const packedWithUndefined = action.pack({
  itemId: '123',
  deliveryMethod: undefined,
})

console.log(packedWithUndefined)
action.unpack(packedWithUndefined)

Actual behavior

pack() serializes the optional enum as -1. Then unpack() throws:

Error: Invalid index -1 for enum 'deliveryMethod'

By contrast, this round-trip succeeds:

const packedWithoutProperty = action.pack({itemId: '123'})
console.log(action.unpack(packedWithoutProperty))

Expected behavior

Explicit undefined for an optional field should ideally be treated the same as an omitted property. Alternatively, pack() should reject it immediately with a clear error instead of producing a payload that unpack() cannot deserialize.

This is related to, but distinct from, #3: that issue concerns warning output for a valid omitted optional field, while this one produces an invalid payload and a runtime exception.