lisa.analysis.tasks: Fix plot_tasks_activation() with trace window
lisa.analysis.tasks: Fix plot_tasks_activation() with trace window

FIX

Using a trace window can remove all activations for a given task. That
leads to <NA> values in dataframes, that bokeh chokes on. Avoid creating
such <NA> values in the first place fixes that.

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