fix(email): Emails are not sent in multiple cases

- fix(email): transactional emails are all not sent if `Use system wide (transactional) email settings` is disabled and no other email provide is setup on the Notifications page
- fix(email): no emails are sent if SMTP username and SMTP password are empty (which is the case in dev for example)
- fix(email): Wrong test email notification is used, causing the transactional email test notification to fail if no email provider is set up on the Notifications page.

app/Notifications/TransactionalEmails/InvitationLink.php

@@ -16,9 +16,10 @@ class InvitationLink extends CustomEmailNotification
         return [TransactionalEmailChannel::class];
     }
 
-    public function __construct(public User $user)
+    public function __construct(public User $user, public bool $isTransactionalEmail)
     {
         $this->onQueue('high');
+        $this->isTransactionalEmail = true;
     }
 
     public function toMail(): MailMessage