Fixed: invalid menu item validator was not checked properly

Previously, if CPApplication -targetForAction:to:from: returned nil, no check was done to see if that failed because there was no action or no valid target. If the item had no item, no target, and to action binding, it would be left enabled, which was incorrect.

This commit fixes the validation to disable the item if targetForAction:to:from: returns nil and the item has an action or target.