fix: PartialGuildMember#id is not null

[suneettipirneni]

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

Closes #6839, This seems to fix a regression introduced in #6066

The docs say a user object is always provided in these events. So it shouldn't be nullable.

I also converted some empty interfaces into types.

Status and versioning classification:

Please move lines that apply to you out of the comment:

• 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

[DTrombett]

You're right! I didn't notice that user isn't nullable in my PR.
On another note, why are you converting interfaces to types? This isn't possible in PartialDMChannel where a property is overwritten and I'm pretty sure this wasn't done with other structures for this future possibility too.

[suneettipirneni]

On another note, why are you converting interfaces to types? This isn't possible in PartialDMChannel where a property is overwritten and I'm pretty sure this wasn't done with other structures for this future possibility too.

Thanks for pointing this out, this has been reverted.