BUG: plot_locs and plot_data when no electrodes pass the filter threshold

When all electrodes have kurtosis values greater than the threshold, plot_locs and plot_data both fail.

plot_data fix: when bo.get_locs() is empty, plot_data should return an empty plot
plot_locs fix:
- if there are no electrodes, plot_locs should show an empty glass brain plot
- if there are electrodes, but none that pass the threshold, i think we should indicate "bad" electrodes in some additional color-- e.g. as x's or very light gray, or something along those lines.