Fix empty name value resulting in non closed tag

See https://github.com/mapbox/carto/commit/5938ebb6097809ecab103936cfd0100fb74f9755#commitcomment-4130230

When text-name value was empty, so tag was not closed.

If you wonder why one would ever use an empty value, here you have an example: https://github.com/hotosm/HDM-CartoCSS/blob/master/labels.mss#L286 :)

One test included for same price :)

Thanks!

Yohan