Fix mailables always being queued for later if using Queueable trait (#…

…25378)

src/Illuminate/Mail/Mailable.php

@@ -157,7 +157,7 @@ public function send(MailerContract $mailer)
      */
     public function queue(Queue $queue)
     {
-        if (property_exists($this, 'delay')) {
+        if (isset($this->delay)) {
             return $this->later($this->delay, $queue);
         }