Links to "#" get rewritten to just "" which causes a refresh if clicked

[jakemac53]

https://gist.github.com/jakemac53/ff1dc6c1ff4ae6e72cdf

It is fairly common for people to use an href of "#" to style something like a link but make it not actually link anywhere. If this is done inside a polymer element though, then it gets rewritten to just an empty href which causes a full page refresh when clicked.

[sorvell]

Unfortunately, we're assuming we can rely on the hash property of the URL object, but apparently we cannot. See this, for example: http://jsbin.com/topil/1/edit.

We'll need to specifically handle the case when a hash exists but is empty.

[sorvell]

@azakus Please review the PR. Thanks.