handle possible null actions in interactive.py (#3914)

* handle possible null actions interactive.py

It is possible that there are tasks, but there is no previous actions value so actions is None. This addresses that issue by setting actions to an empty string if it is None.

src/dispatch/plugins/dispatch_slack/incident/interactive.py

@@ -1531,6 +1531,7 @@ def handle_report_tactical_command(
 
     incident = incident_service.get(db_session=db_session, incident_id=context["subject"].id)
     if incident.tasks:
+        actions = "" if actions is None else actions
         actions += "\n\nOutstanding Incident Tasks:\n".join(
             [
                 "-" + task.description