Fix for an empty hash

This wasn't behaving right for certain links on my app. Specifically under these circumstances:
- the current page has a non-empty hash, say `"#15/42.3956/-83.0854"`
- the clicked pjax-enabled link has `href="#"`

You'd expect this to be covered by the "ignore anchors on the same page" clause, BUT with only a `#` on your link, the value of `link.hash` is actually an empty string and we pass right by the desired `return`! Anyway, perhaps I've missed something but it works better for me now, and here I am forking and submitting this idea for your consideration.