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=…

app/assets/javascripts/edit/id.js.erb

@@ -4,7 +4,8 @@ $(document).ready(function () {
   var id = $("#id-embed");
 
   if (id.data("configured") === true) {
-    var hashParams = OSM.params(location.hash.substring(1));
+    var hash = location.hash.substring(1);
+    var hashParams = hash ? OSM.params(hash) : {};
     var mapParams = OSM.mapParams();
     var params = {};