fix($location): allow empty string URLs to reset path, search, and hash

Currently, providing '' to $location#url will only reset the hash, but otherwise has no effect. This
change brings the behaviour of $location#url more inline with window.location.href, which when
assigned to an empty string loads the page's base href.

Before:
$location.url() // http://www.example.com/path
$location.url('') // http://www.example.com/path

After:
$location.url() // http://www.example.com/path
$location.url('') // http://www.example.com

Fixes #10063