[FIX] mail: prevent crash when using channel palette before init

`joinChannel` expects messaging to be initialized (in particular to
access store user) but doesn't actually wait for it.

Part-of: odoo/odoo#144580

addons/mail/static/src/core/common/thread_service.js

@@ -535,6 +535,7 @@ export class ThreadService {
     }
 
     async joinChannel(id, name) {
+        await this.env.services["mail.messaging"].isReady;
         await this.orm.call("discuss.channel", "add_members", [[id]], {
             partner_ids: [this.store.user.id],
         });