Allow single slash, but not empty string url prefix

[jfinkels]

In pull request #500 we allowed the empty string as a URL prefix. However, this conflicts with werkzeug's Rule class, which expects the URL prefix to be a string that begins with a slash. Although this does not cause a problem with the code as it is now, this did cause me a problem while I was updating the code to create a schema for issue #625.

We need to change this so that the user cannot enter empty strings.