Fix handling of empty paths in AbstractSitePathUrlFactory.

The formatPath(String path) method in the AbstractSitePathUrlFactory class
didn't handle an empty path (empty String) correctly. It would return //
(two slashes), instead of just one.

Fixes MOBILE-113.
Merge branch 'evpaassen-MOBILE-113'

* evpaassen-MOBILE-113:
  Fix handling of empty paths in AbstractSitePathUrlFactory.