docs(ChannelManager): fetch can return Promise<null>

[advaith1]

Please describe the changes this PR makes and why it should be merged:
ChannelManager#fetch can return Promise<null> but is only documented to return Promise<Channel>, this PR fixes the docs and typings.

fetch:

discord.js/src/managers/ChannelManager.js

Line 92 in d744e51

return this.add(data, null, cache);

add:

discord.js/src/managers/ChannelManager.js

Lines 32 to 35 in d744e51

	if (!channel) {
	this.client.emit(Events.DEBUG, `Failed to find guild, or unknown type for channel ${data.id} ${data.type}`);
	return null;
	}

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 only includes non-code changes, like changes to documentation, README, etc.