[FIX] mail: do not return when processing notifications

Before this commit, the `mail.channel/seen` notification handler
would have returned if the channel was unknown.

This is incorrect since it would prevent other notifications from
the same batch from being processed. This commit fixes the issue.

closes odoo/odoo#116518

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/mail/static/src/core/messaging_service.js

@@ -351,7 +351,7 @@ export class Messaging {
                     if (!channel) {
                         // for example seen from another browser, the current one has no
                         // knowledge of the channel
-                        return;
+                        continue;
                     }
                     if (this.store.user.id === partner_id) {
                         channel.serverLastSeenMsgBySelf = last_message_id;