fix: Fix downloading a playlist as a TAR archive

This is the only time where youtube-dl returning an empty URL is not a problem.

classes/EmptyUrlException.php

@@ -0,0 +1,15 @@
+<?php
+/**
+ * EmptyUrlException class.
+ */
+
+namespace Alltube;
+
+use Exception;
+
+/**
+ * Exception thrown when youtube-dl returns an empty URL.
+ */
+class EmptyUrlException extends Exception
+{
+}