Prevent empty strings to curl if no auth

If auth is empty, then empty quotes are passed to curl, resulting in an
error of `curl: (3) <url> malformed`. Basically the same as:

```
$ curl google.com ""
<HTML><HEAD><meta http-equiv="content-type" content="text/html;charset=utf-8">
<TITLE>302 Moved</TITLE></HEAD><BODY>
<H1>302 Moved</H1>
The document has moved
<A HREF="http://www.google.ie/?gws_rd=cr&amp;ei=jjxAVMHbAuLN7Qbyg4HgCQ">here</A>.
</BODY></HTML>
curl: (3) <url> malformed
```