fix: typing mismatch for empty strings being treated as undefined

[WhatCats]

Problem

At the moment methods like GuildMemberManager:fetch that have an optional string parameter are doing a falsy check for if the parameter was omitted. But consider the following typescript code:

const id: string = ""
const member: GuildMember = await guild.members.fetch(id)

According to the typings member should be a GuildMember but its actually a Collection<Snowflake, GuildMember> because the empty string is falsy (this is why we love JS).

My Suggestion

Do more exact undefined/null checks.
(I might not have found every place this is an issue)

[vercel]

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[almeidx]

Personally, I think this is a step in the right direction. However, I don't think this change alone is enough, as it will potentially bring different issues. For instance, passing an empty string to RoleManager#fetch() will now make a request to the Get Guild Roles endpoint, but through the Get Guild Role path internally. This will mean the endpoint will return a different body than expected, which might create worse errors that are harder to pinpoint.

To fix that, we should make further changes here. For instance, modifying the resolveId() method of managers to return null for empty strings. This solves the issue I mentioned in most places, but there are still some places that do not make use of this method (e.g., the RoleManager), so those places would have to be modified to make use of it.