Do not issue warning when sitemenu item has no link

The menu editor in the Channel Manager has three config options for menu items: internal, external link and "None". When you configure the None option, you get warnings about this from HST in the log. They seem out of place now. This warning should not be given.

WARN  http-nio-8080-exec-3 [HstSiteMenuItemImpl.getHstLink:147] Sitemenu item 'none' of menu 'main' does not contain an hstSiteMapItemRefId, an hstSiteMapItemPath or an externalLink. Cannot create link for sitemenu item, return null