MINOR: event_hdl: add event_hdl_sub_list_empty() helper func

MINOR: event_hdl: add event_hdl_sub_list_empty() helper func

event_hdl_sub_list_empty() may be used to know if the subscription list
passed as argument is empty or not (ie: if there currently are any
subcribers or not). It can be useful to know if the subscription is empty
is order to avoid unecessary preparation work and skip event publishing to
save CPU time if we already know that no one is interested in tracking the
changes for a given subscription list.