The idea is to extend both spo navigation node add and spo navigation node set to support linkless navigation nodes.
In the UI this is also called label.

I suggest that:

• url option becomes optional for spo navigation node add
• url can accept an empty string value for spo navigation node set

When this is the case, we can create a linkless navigation node by simply adding http://linkless.header/ as link for the node.