feat(GuildMember): make GuildMember#setNickname first param nullable

[zaida04]

Passing null into GuildMember#setNickname will reset the nickname. But neither the typings nor the jsdoc indicate that you can pass that in.

Status

• 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

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.

[zaida04]

In response to #5068

[SpaceEEC]

This also need to be documented here:

discord.js/src/structures/GuildMember.js

Line 277 in 105238f

* @property {string} [nick] The nickname to set for the member

And here:

discord.js/typings/index.d.ts

Line 2651 in 105238f

nick?: string;