N/A to None. Revert validates from validates_presence_of.

app/models/ci/trigger_schedule.rb

@@ -10,7 +10,7 @@ module Ci
 
     delegate :ref, to: :trigger, allow_nil: true
 
-    validates_presence_of :trigger
+    validates :trigger, presence: { unless: :importing? }
     validates :cron, cron: true, presence: { unless: :importing? }
     validates :cron_timezone, cron_timezone: true, presence: { unless: :importing? }
     validates :ref, presence: { unless: :importing? }

app/views/projects/triggers/_trigger.html.haml

@@ -33,7 +33,7 @@
     - if trigger.trigger_schedule.persisted?
       = trigger.trigger_schedule.real_next_run
     - else
-      N/A (External trigger)
+      None (External trigger)
 
   %td.text-right.trigger-actions
     - take_ownership_confirmation = "By taking ownership you will bind this trigger to your user account. With this the trigger will have access to all your projects as if it was you. Are you sure?"