Fix: Calling io.emit("event") with events that have no arguments results in TypeScript errors

[ZachHaber]

The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Typescript type fixe relating to #4817, also type failures in the tests due to the client#id type becoming string|undefined (f9c16f2)

Current behavior

import { Server } from "socket.io";

interface ServerToClientEvents {
  noArg: () => void;
}

const io = new Server<{}, ServerToClientEvents>();

io.emit("noArg"); // <- Error

New behavior

import { Server } from "socket.io";

interface ServerToClientEvents {
  noArg: () => void;
}

const io = new Server<{}, ServerToClientEvents>();

io.emit("noArg"); // ✔️ - no error!

Other information (e.g. related issues)

#4914

[lts20050703]

Hmm does this also fix #4917?

[ZachHaber]

Yes, they all use the same utility type to set up which events are allowed. I just added in a test I was missing for a namespace emit with noArgs.

[darrachequesne]

@ZachHaber thanks 👍