[Backport 3.1] LocationMapTreeComponentDocuments#add2LocationMap throws NPE when sitemap parameters object contains a null key or value

The problem:
In HstSiteMapItemService parameters are stored in a HashMap : A HashMap allows null as key or value
In LocationMapTreeComponentDocuments an iteration through HstSiteMapItemService parameters is done as follows:

final Properties siteMapItemParameters = new Properties();
for (Map.Entry<String, String> param : siteMapItem.getParameters().entrySet()) {
    siteMapItemParameters.put(param.getKey(), param.getValue());
}

problem now is that Properties is a subclass op Hashtable which does throw an NPE on a null key or value.