fix: ParentNamespace adapter broadcast bug

[gkt5015]

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

Current behavior

When a dynamic namespace emits to an individual socket, it will be met with an error:

node_modules/socket.io/dist/parent-namespace.js:88
        this.children.forEach((nsp) => {
                      ^

TypeError: Cannot read properties of undefined (reading 'forEach')
    at ParentBroadcastAdapter.broadcast (node_modules/socket.io/dist/parent-namespace.js:88:23)
    at BroadcastOperator.emit (node_modules/socket.io/dist/broadcast-operator.js:169:26)
    at Socket.<anonymous> (server.js:60:33)
    at Socket.emit (node:events:520:28)
    at Socket.emitReserved (node_modules/socket.io/dist/typed-events.js:56:22)
    at Socket._onclose (node_modules/socket.io/dist/socket.js:547:14)
    at Client.onclose (node_modules/socket.io/dist/client.js:247:20)
    at Socket.emit (node:events:532:35)
    at Socket.onClose (node_modules/engine.io/build/socket.js:304:18)
    at Object.onceWrapper (node:events:639:28)

New behavior

This fixes that error and adds a test that would guard this scenario.

Other information (e.g. related issues)

resolves #4985

[darrachequesne]

@gkt5015 awesome, thanks for the pull request 👍