Navigation block should not show elements which are both empty and have no link

It's currently possible an element in the navigation block to have no link, and have children but not display them.
The result is a node in the navigation tree which has no link and cannot be expanded rendering it a digital paper-weight on our screens.

In this scenario, we should not display the link as it's potentially confusing - it doesn't do anything.

To replicate:

• Open the site frontpage (or most other pages)
• Turn editing on
• Edit the block settings for the navigation block
• Change the "Generate navigation for the following" setting to "Categories and courses" and Save changes
• Notice that the "Site pages" link can't be expanded any more and don't link to anything

Proposal: Not show the bleeding thing