#1 Only write a legend entry for plots with non-empty labels

[Keith]

Modified Legend.Draw to ignore plots with empty labels.
This is useful, for example, when you add a TextPlot to
a LinePlot to label points on the LinePlot (both
plotting the same data) and you don't want the TextPlot
to show up in the legend. If none of the plots have
labels, the legend isn't drawn at all.

This patch is against Legend.cs v1.13