This is an action function that does nothing when the corresponding milestone
is triggered. When the listener is monitoring a trial and determining the
time to trigger a milestone, data is automatically locked with other necessary
data manipulation being executed. If the users have no intent to modify the
trial adaptively at the milestone, e.g., adding (Trial$add_arms()) or
removing (Trial$remove_arms()) arm(s),
changing sampling ratio(s) (Trial$update_sample_ratio()),
modifying trial duration, carrying out statistical testing,
or saving intermediate results (Trial$save()), then this function
can be used to set the argument action when creating a new milestone.