It is a known limitation that Content Library Service does not synchronize empty library items.
An empty library item should not result if you use the vSphere Web Client to import a library item. An empty library item comes up when using the Content Library API, because importing an item to a content library involves multiple steps. First, you create a library item, which will initially have no content. Then, you upload files to the library item using the UpdateSession class. If you don't upload content to the library item, the library item remains empty.