VDEF doesn't show up in the legend

The GPRINT of a VDEF doesn't have a format. For example:
> "GPRINT:avg:LAST:%5.1lf%s Last"
> "GPRINT:tot:%5.1lf%s Total\l"

It didn't show up in the legend. Firebug showed variable format isn't
undefined, but an empty string in case of VDEF. So just check for falsy.