fix(Webhook): check whether user is null during patching

[rockerbacon]

Please describe the changes this PR makes and why it should be merged:
While fetching a webhook using the Client#fetchWebhook method, it is possible for the Discord API to send a response with some properties set to null and others not defined.
While creating a new instance of the Webhook class, the _patch method will check whether or not a property was returned using the in operator. An issue then arises when the property "user" is null: the check 'user' in data evaluates to true (the property is indeed in data, it's just set to the value "null") and a null value is then sent to CachedManager#_add, which attempts to access the property id in a null value.

The following is the relevant stack trace of the error I encountered:

TypeError: Cannot read properties of null (reading 'id')
    at UserManager._add (/home/loka/Documents/discord-bots/greatreads-discord-bot/node_modules/discord.js/src/managers/CachedManager.js:47:48)
    at Webhook._patch (/home/loka/Documents/discord-bots/greatreads-discord-bot/node_modules/discord.js/src/structures/Webhook.js:91:39)
    at new Webhook (/home/loka/Documents/discord-bots/greatreads-discord-bot/node_modules/discord.js/src/structures/Webhook.js:25:20)
    at Client.fetchWebhook (/home/loka/Documents/discord-bots/greatreads-discord-bot/node_modules/discord.js/src/client/Client.js:313:12)

And this is the offending Discord API response, obtained by logging the immediate response (Client.js ln312) to the console:

{
  data: {
    application_id: '<redacted>',
    avatar: null,
    channel_id: null,
    guild_id: null,
    id: '1184125670163488828',
    name: '<redacted>',
    type: 3,
    user: null
  }
}

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

2 Ignored Deployments

Name	Status	Preview	Comments	Updated (UTC)
discord-js	⬜️ Ignored (Inspect)	Visit Preview		Dec 13, 2023 11:38pm
discord-js-guide	⬜️ Ignored (Inspect)	Visit Preview		Dec 13, 2023 11:38pm

[Jiralite]

Pretty sure this is an issue with the API. The user of a webhook is not documented to be null.

We'll see what happens with discord/discord-api-docs#6576.

[rockerbacon]

Thanks for the quick response.

If it is of any relevance: my use case is editing a message sent by my own bot at the end of an OAuth2 authorization process. Since the authorization system is designed to be a separate distributed system, our strategy was to store the message id, webhook id and webhook token in a central database and retrieve them later.
I worked around the issue by instantiating a WebhookClient directly, as documented in the discord.js guide.

Let me know if you'd require any further assistance with the issue.

[Qjuh]

You could also just instantiate an InteractionWebhook instance directly, no need to fetch it or make another Client (which your WebhookClient is) if you already have a logged in Client.

[Jiralite]

Looks like we got an official response: discord/discord-api-docs#6576 (comment)

Fetching these kinds of webhooks should return a 404, which means we shouldn't be encountering a null user. I'll be closing this.