PR #1119 (adding support for `highway=motorway_junction`) missed a few cases where NULLIF functions were needed in order to suppress empty-string `subclass` tags from appearing in vector tiles, resulting in unnecessary `subclass` entries on `transportation_name` objects where it was not needed, for example:  This PR adds the missing NULLIF function so that those empty tags are suppressed: 
transportation_name
Docs
Read the layer documentation at http://openmaptiles.org/schema#transportation_name

