Short-circuit Project#auto_devops_variables

This makes Project#auto_devops_variables more performant by evaluating
the application settings only if necessary.

app/models/project.rb

@@ -1604,15 +1604,13 @@ class Project < ActiveRecord::Base
   def auto_devops_variables
     return [] unless auto_devops_enabled?
 
-    variables = []
-
-    if current_application_settings.auto_devops_domain.present?
-      variables << { key: 'AUTO_DEVOPS_DOMAIN',
-                     value: current_application_settings.auto_devops_domain,
-                     public: true }
-    end
-
-    auto_devops&.variables || variables
+    auto_devops&.variables || if current_application_settings.auto_devops_domain.present?
+                                [{ key: 'AUTO_DEVOPS_DOMAIN',
+                                   value: current_application_settings.auto_devops_domain,
+                                   public: true }]
+                              else
+                                []
+                              end
   end
 
   def append_or_update_attribute(name, value)