Absent labels are rendered as "undefined" instead of ignored

[herebebeasties]

I'd like to optionally label my nodes, but if the first (any?) node has a label then any other nodes without a label property set have their label rendered as "undefined".

There's a workaround for this, which is to set nodes which you don't want to have labels on as label: "" but that's a bit unexpected.

[ericdrowell]

I will fix this as well over the weekend. Thanks!