lisa.analysis.tasks: Best effort in plot_tasks_activations()
lisa.analysis.tasks: Best effort in plot_tasks_activations()

FIX

When no explicit list of tasks is passed to plot_tasks_activations(), do
not bail out at the first empty dataframe.

ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa,credp/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa