types(webhook): avatar can be null

[DTrombett]

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

Webhook#avatar can be null if the webhook hasn't any avatar. The documentation is correct, and Webhook#avatarURL() handles it correctly, but types were missing the null type.

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
• This PR only includes non-code changes, like changes to documentation, README, etc.

[vercel]

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

1 Ignored Deployment

Name	Status	Preview	Updated
discord-js	⬜️ Ignored (Inspect)		Sep 2, 2022 at 3:22PM (UTC)

[codecov]

Codecov Report

Merging #8541 (f3ceba8) into main (3b7ba40) will increase coverage by 0.07%.
The diff coverage is n/a.

❗ Current head f3ceba8 differs from pull request most recent head 2690637. Consider uploading reports for the commit 2690637 to get more accurate results

@@            Coverage Diff             @@
##             main    #8541      +/-   ##
==========================================
+ Coverage   86.35%   86.43%   +0.07%     
==========================================
  Files          86       87       +1     
  Lines        8598     8550      -48     
  Branches     1102     1105       +3     
==========================================
- Hits         7425     7390      -35     
+ Misses       1131     1117      -14     
- Partials       42       43       +1     

Flag	Coverage Δ
builders	100.00% <ø> (ø)
collection	100.00% <ø> (ø)
proxy	73.28% <ø> (-0.82%)	⬇️
rest	91.88% <ø> (-0.13%)	⬇️
utilities	100.00% <ø> (?)
voice	64.08% <ø> (+0.21%)	⬆️
ws	60.22% <ø> (-0.24%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/proxy/src/handlers/proxyRequests.ts	69.49% <0.00%> (-2.16%)	⬇️
packages/rest/src/lib/errors/DiscordAPIError.ts	95.32% <0.00%> (-1.14%)	⬇️
packages/ws/src/ws/WebSocketShard.ts	22.41% <0.00%> (-0.96%)	⬇️
.../src/strategies/sharding/SimpleShardingStrategy.ts	77.63% <0.00%> (-0.58%)	⬇️
packages/voice/src/VoiceConnection.ts	81.28% <0.00%> (-0.48%)	⬇️
packages/voice/src/audio/AudioPlayer.ts	80.55% <0.00%> (-0.40%)	⬇️
...trategies/context/WorkerContextFetchingStrategy.ts	91.83% <0.00%> (-0.33%)	⬇️
.../src/strategies/sharding/WorkerShardingStrategy.ts	95.81% <0.00%> (-0.08%)	⬇️
packages/ws/src/ws/WebSocketManager.ts	95.62% <0.00%> (-0.08%)	⬇️
packages/voice/src/DataStore.ts	98.11% <0.00%> (-0.04%)	⬇️
... and 45 more

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more