lisa.analysis.tasks: Make df_tasks_total_residency robust against missing tasks
lisa.analysis.tasks: Make df_tasks_total_residency robust against missing tasks

Ignore missing tasks, since that's a common occurence, e.g. if the task
is outside the current TraceView window.

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