[FIX]mail: fix help command in livechat with unlogged user

When an unlogged user creates a livechat, typing the /help command in the
backend would result in the help command sending a traceback.
The function processing the command would assume the existence of a channel
partner name, which is not the case when the user starting the chat is unlogged.

opw:1835484

addons/mail/models/mail_channel.py

@@ -648,7 +648,7 @@ class Channel(models.Model):
                 msg += _(" This channel is private. People must be invited to join it.")
         else:
             channel_partners = self.env['mail.channel.partner'].search([('partner_id', '!=', partner.id), ('channel_id', '=', self.id)])
-            msg = _("You are in a private conversation with <b>@%s</b>.") % channel_partners[0].partner_id.name
+            msg = _("You are in a private conversation with <b>@%s</b>.") % (channel_partners[0].partner_id.name if channel_partners else _('Anonymous'))
         msg += _("""<br><br>
             You can mention someone by typing <b>@username</b>, this will grab its attention.<br>
             You can mention a channel by typing <b>#channel</b>.<br>