Backport : ResolvedSiteMapItemImpl - throws NPE when proccessing url with _default_

Steps to reproduce:

• Create a sitemap with a wildcard .
• Add sitemap parameter with params (like ${1}) to the sitemap wildcard
• In a browser type in url that matches the just created sitemap. Use for the wildcard default. Example: http://localhost:8080/site/test/_default_

See attached a screenshot of an example how to setup the configuration and the stack trace.

Actual output:
500 error page because of the NPE

Expected output:
404 error page

Workaround:
Add redirect rule in Apache Httpd to redirect all the urls with default to a 404 error page