DEV: Fix RSpec service matchers when a model is not found

This is a follow-up of d749227e87.

This patch checks if the key `not_found` is present on the result object
instead of calling `#blank?` on the model, as it can trigger an
`ActiveRecord` relation.

spec/support/service_matchers.rb

@@ -99,7 +99,7 @@ module ServiceMatchers
     end
 
     def step_failed?
-      super && result[name].blank?
+      super && result[step].not_found
     end
   end