Gallery folders without gallerytype still has "Add Image" option

If a gallery folder a no values for the hippostd:gallerytype, the context menu still shows "Add Image" option.

Using the upload dialog leads to an unfriendly error, saying to try again.
See .

The goal is to have folders in which only subfolders can be created.

Relevant class is org.hippoecm.frontend.plugins.gallery.GalleryWorkflowPlugin.