fix(RESTPostAPIGuildsJSONBody): make some fields nullable

[cherryblossom000]

This PR makes afk_channel_id and system_channel_id in RESTPostAPIGuildsJSONBody and parent_id in APIGuildCreatePartialChannel nullable.

These types are used in the following:

• RESTPostAPIGuildsJSONBody: I tested creating a guild using POST /guilds with a null afk_channel_id and system_channel_id, and a channel with a null parent_id, and it worked.
• APITemplate (via APITemplateSerializedSourceGuild): The example template object in the docs includes a null afk_channel_id and a null parent_id for a channel. I also tested creating a template from a guild without a system channel and the system_channel_id in the returned template object was null.
• RESTPostAPIGuildChannelJSONBody: I tested creating a channel using POST /guilds/:id/channels with a null parent_id in the JSON body, and it worked.

[vladfrangu]

LGTM!