I ran into a similar problem when using contour plots, which can also be empty.
The solution I came up with is:
clear all
figure, hold all
fig{1} = contour(rand(10), [0.5 0.5], 'r');
fig{2} = contour(rand(0), [0.5 0.5], 'g');
fig{3} = contour(rand(10), [0.5 0.5], 'b');
name = {'fig1', 'fig2', 'fig3'};
used = {};
for k = 1 : numel(fig)
if ~isempty(fig{k})
used = {used{:}, name{k}};
end
end
legend(used)
