fix: mark channel name as nullable

[onerandomusername]

Group Direct Message channels can have a nullable name, which is exposed to bots via fetching an invite that goes to a group DM.

[Jupith]

pretty sure they don't document any quirks belonging to group dms since their considered deprecated

[onerandomusername]

pretty sure they don't document any quirks belonging to group dms since their considered deprecated

Group DMs are still exposed to bots when fetching an invite that goes to a group dm, which to my knowledge are not deprecated. These invites can be sent in servers, which means that a bot that filters invites by their targets is not expecting a null name for the invite's channel.

[SinisterRectus]

See #1639 for the same rejected change(s) and comment from night.

[onerandomusername]

There's numerous code review comments which have not been addressed, and given managed group dms are somewhat deprecated and not really used I don't imagine there's much use to documenting these changes anyhow.

These aren't managed group dms, though. Group DM channels are fetchable by any bot, using a group dm invite.