fix(BaseClient): Fall back to userAgentAppendix

[Jiralite]

Please describe the changes this PR makes and why it should be merged:
We're currently getting an undefined string in our user agent strings (if an appendix is not specified):

Request {
  headersTimeout: undefined,
  bodyTimeout: undefined,
  throwOnError: false,
  method: 'GET',
  abort: null,
  body: null,
  completed: false,
  aborted: false,
  upgrade: null,
  path: '/api/v10/gateway/bot',
  origin: 'https://discord.com',
  idempotent: true,
  blocking: false,
  reset: null,
  host: null,
  contentLength: null,
  contentType: null,
  headers: 'User-Agent: DiscordBot (https://discord.js.org, 2.2.0) undefined\r\n' +
    'Authorization: Bot [REDACTED]\r\n',
  expectContinue: false,
  servername: null,
  [Symbol(handler)]: RequestHandler {
    responseHeaders: null,
    opaque: null,
    callback: [Function (anonymous)],
    res: null,
    abort: null,
    body: null,
    trailers: {},
    context: null,
    onInfo: null,
    throwOnError: undefined,
    highWaterMark: undefined,
    [Symbol(async_id_symbol)]: 92,
    [Symbol(trigger_async_id_symbol)]: 0,
    [Symbol(kSignal)]: AbortSignal { aborted: false },
    [Symbol(kListener)]: [Function (anonymous)]
  }
}

This happens because userAgentAppendix may be explicitly undefined here:

discord.js/packages/discord.js/src/client/BaseClient.js

Lines 42 to 48 in 278396e

	rest: {
	...defaultOptions.rest,
	...options.rest,
	userAgentAppendix: options.rest?.userAgentAppendix
	? `${Options.userAgentAppendix} ${options.rest.userAgentAppendix}`
	: undefined,
	},

Eventually, it reaches this point:

discord.js/packages/rest/src/lib/REST.ts

Line 79 in 278396e

this.options = { ...DefaultRestOptions, ...options };

The explicit undefined value is overwriting the default value.

This is a fault of discord.js as it is not possible to do new REST({ userAgentAppendix: undefined }); due to exact optional property types being set. I've thus opted to fall back to the default value which has been tested even with the past bug #9421 and works fine.

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

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