[5.8] Bug Fix: Render MailMessage if view method was used

[muhammetsait]

MailMessage supports customizing its contents by setting lines of text or by using the view method to specify a custom template.

However, if you call view on MailMessage it will fail when calling render.

Steps to reproduce:

• Create a notification php artisan make:notification InvoicePaid.

• In the method toMail: return a new MailMessage with a custom view:

  public function toMail($notifiable)
  {
      return (new MailMessage)->view(
          'emails.name', ['invoice' => $this->invoice]
      );
  }

• Add code to preview that notification:

  Route::get('mail', function () {
      $invoice = App\Invoice::find(1);
  
      return (new App\Notifications\InvoicePaid($invoice))
                  ->toMail($invoice->user);
  });

It will result in the following error: InvalidArgumentException View [] not found.

All code snippets were taken from https://laravel.com/docs/5.8/notifications
Specifically, formatting-mail-messages and previewing-mail-notifications sections.