Make guild optional on Invites
Make guild optional on Invites

Make the `guild` structfield on `Invite` and `RichInvite` optional.

This was done due to a change in the [docs].

[docs]: https://github.com/discordapp/discord-api-docs/commit/bc0a15bd11db72644633080903171fbc3e71b026

zeyla/serenity.rs,acdenisSK/serenity,Flat/serenity,Lakelezz/serenity,zeyla/serenity