1. Open ContentTypesTool (The ContentTypes contextual ribbon is displayed)
2. Ctrl + Click on the content selected to remove it from the selection and have an empty selection
3. The ContentTypes contextual ribbon disapears

or

1. Open ContentTypesTool (The ContentTypes contextual ribbon is displayed)
2. Click on the filter textfield
3. Type the beginning of an existing content type (for instance type "Arti")
4. As the selection is cleared, the ContentTypes contextual ribbon disapears

I think the contextual ribbon must be contextual to the tool and not to a content type, because some buttons do not depends on the selection (Hierarchical view, zip export, etc.)