SAK-26139 Don't attempt to lookup aliases for null.

The timeout handler in the portal doesn't have a active site and so asks the portal to render a response with a null site, the portal tries to lookup aliases for null which will never work. The SiteService has a null check shortcut but the alias service doesn't. Adding shortcut to prevent nulls getting to the alias service.

git-svn-id: https://source.sakaiproject.org/svn/portal/trunk@308505 66ffb92e-73f9-0310-93c1-f5514f145a0a
SAK-26139 SiteNeighbourhoodService.parseSiteAlias is documented as returning null
 
 Although we can create a reference and lookup a site with null it's never going to work, so just shortcut out.

git-svn-id: https://source.sakaiproject.org/svn/portal/trunk@308506 66ffb92e-73f9-0310-93c1-f5514f145a0a