fix(MessagePayload): prevent spread of undefined

[kyranet]

Please describe the changes this PR makes and why it should be merged:

When passing attachments: [] to MessageOptions and no files (to clear all attachments in a message edit), this code would result on the following logic to be executed:

const attachments = this.options.files?.map((file, index) => ({
  id: index.toString(),
  description: file.description,
}));
if (Array.isArray(this.options.attachments) /* true */) {
  this.options.attachments.push(...attachments /* undefined */);
} else {
  this.options.attachments = attachments;
}

When spreading undefined, a TypeError is thrown with the message undefined is not iterable (cannot read property undefined) on Node.js v16.3.0.

Defaulting attachments to an empty array in that branch resolves the issue with the minimal impact possible.

Status and versioning classification:

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