Set default endpoint config to null

The S3Client does not require an endpoint attribute by default. Passing null is the most sensible default to get this package working out of the box.

src/config/cdn.php

@@ -110,7 +110,7 @@
                 | https://www.digitalocean.com/community/questions/how-to-use-digitalocean-spaces-with-the-aws-s3-sdks?answer=39594
                 |
                 */
-                'endpoint' => '',
+                'endpoint' => null,
 
                 /*
                 |--------------------------------------------------------------------------