Merge pull request #4 from pommi/master

fix javascript error with VDEF resource(s)
RrdGraph.js: gprint: fix check for undefined format in case of vdef

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.