When using unrequired node reference as controlling field, cannot trigger controlled field based on 'empty' selection

Issue Summary

This module is great, thanks for the contribution.

We ran into a problem where we want to trigger the appearance of a textfield if a node reference field is left unselected.

Normally, when someone goes to create/edit a node of this type, there is a drop down listing of specific node types for the node reference field. However, we don't require this field, so the user can select - None - from the dropdown instead of a node to reference.

This module does not account for that. When I configure how the triggered field should be shown, I can't trigger it based off the - None - value selected for the node reference.

Brian