Fix alias being nil corner case

When the node's crowbar/display/alias was nil, we'd get a gsub with nil
later in the template. Force a conversion into an empty string.
See https://github.com/aspiers/barclamp-crowbar/commit/b08a744df25f2681ff2126240997c5aedc44301c#diff-496ddf4ffc5694a5fcdddbff8206dc8dR259