Allow skipping empty event group levels

This minimalistic patch makes sure that any empty event group levels
can be left out of the configuration file instead of requiring an
empty array in that place.

app/controllers/miq_policy_controller/alerts.rb

@@ -477,10 +477,10 @@ def alert_build_pulldowns
       @sb[:alert][:events] ||= {}
       EmsEvent.event_groups.each do |_k, v|
         name = v[:name]
-        v[:detail].each do |d|
+        v[:detail]&.each do |d|
           @sb[:alert][:events][d] = name + ": " + d if vm_events.include?(d)
         end
-        v[:critical].each do |c|
+        v[:critical]&.each do |c|
           @sb[:alert][:events][c] = name + ": " + c if vm_events.include?(c)
         end
       end