Fix edge case in AbstractListCell

Clear the cell if the item is updated with a null value.

client/src/main/java/envoy/client/ui/listcell/AbstractListCell.java

@@ -38,6 +38,8 @@ public abstract class AbstractListCell<T, U extends Node> extends ListCell<T> {
 		if (!(empty || item == null)) {
 			setCursor(Cursor.HAND);
 			setGraphic(renderItem(item));
+		} else {
+			setGraphic(null);
 		}
 	}