Fix missing_translation override

Simplify and fix override. A missing translation now returns `false`
instead of the text `translation missing`.

The super method requires the `option` parameter, so we opted to keep
the override.

app/helpers/application_helper.rb

@@ -26,13 +26,7 @@ def locale_for(type:, term:, record_class:)
   end
 
   def missing_translation(value, _options = {})
-    case value
-    when FalseClass, NilClass
-      true
-    when String
-      value.include?('translation missing')
-    else
-      raise "Unexpected value #{value.inspect}"
-    end
+    return true if value.try(:false?)
+    false
   end
 end