Fix asset editors crashing when the container has no site

[jakobploens]

Problem

On Statamic 6, opening any asset editor throws and the editor's fields never render:

Uncaught (in promise) TypeError: can't access property "replaceAll", this.container.site.value is undefined

Assets aren't site-scoped, so the asset publish container has no site. Workspace.initializeEcho() builds the channel name as:

this.channelName = `${reference}.${this.container.site.value.replaceAll('.', '_')}`;

container.site.value is undefined for assets → .replaceAll throws during workspace init (a mounted hook), aborting the editor render. Reproduces on every asset container in a single-site install.

Fix

Build the channel without the site segment when no site is present (and use optional chaining). Entries/terms (which have a site) are unchanged; assets now get a stable, site-less presence channel keyed by their reference instead of crashing.

const reference = this.container.reference.value.replaceAll('::', '.');
const site = this.container.site?.value;
this.channelName = site ? `${reference}.${site.replaceAll('.', '_')}` : reference;

Fixes #126.