fix(Guild): setChannelPositions null parenting

[monbrey]

This fixes a couple of issues introduced in #5507 where not providing a parent to Guild#setChannelPositions would resolve to null, clearing the parent instead of leaving it unchanged.

This also prevented the method from being used to set more than one channel position, due to DiscordAPIError: Only one channel can have a parent_id modified at a time.

Additionally, the typings are updated to accept null in order to move the channel out of the parent category.

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 changes the library's interface (methods or parameters added)

[kyranet]

Nice catch on the parenting issue ^^