generateChannelIdIfNeeded
Generates the channel id based on the member ids if provided channelId is empty. Member-based id should only be used for creating distinct channels. It is a local placeholder: the server names distinct channels differently, so it must never be sent as an id. Member ids are sorted, so the same members always give the same placeholder.
Return
channelId if not blank or new, member-based id.
Parameters
channelId
The channel id. ie 123.
memberIds
The list of members' ids.