only parse hashParams if there is a hash

only parse hashParams if there is a hash

Presumably this was an oversight in the original implementation, as `OSM.params` falls back to the page's `query` string if the function's parameter (here: the page's `hash`) is an empty string.

This fixes some inconsistent behaviour of the website. In the example below, the first URL sets the `background` to `none` while in the second example it doesn't.

* https://www.openstreetmap.org/edit.html?background=none
* https://www.openstreetmap.org/edit.html?background=none#map=…