title: JavaFX: TreeTableView and expanding items without children
body: <p>I have TreeTableView with items. I want that user see the "arrow" to expand even if item has no children. The reason is very simple - lazy load. When user clicks at arrow program checks if there are possible items (for example in DB). If children for this item exist then they are added to item. If they don't exist the "arrow" must disappear. How to do it?</p>