Fix error when a blank url is submitted

This fixes an error introduced in 00664b9d54d79cde1a24bbb6eb5f2d28679739c3 where submitting a blank url would cause an error on [this line](https://github.com/mikesall/charted/blob/00664b9d54d79cde1a24bbb6eb5f2d28679739c3/pub/scripts/PageController.js#L240).