Q: There are cases when content doesn't exist for the top level menu items. In these cases, when the user clicks a top menu item or when the clickItem() method is called on this top menu item, the first child of the menu should be selected. How can I make this happen?

A: This can be done by using the clickItem() method of the component. You need something to qualify the item as selectable or not. In the attached example we used an attribute (dataContent) in the items' xml node, in the component's setup xml file:
...
<item title="Level3 1" contentData="some data"/>
...

If the current item meets your criteria (has the contentData property), you can select it, otherwise you need to check if it has any children and select the first child. If the current item does not have any children, then its' next sibling will be selected. If it doesn't have any siblings either, then the parent's sibling will be selected (only if the item is not on the top level). In the attached file you have the full code, commented.
Files: ATM_SelectNextAvailableItem.zip