Typeahead widget does not trigger onValue event when user clears the field (no selection)

**Bug**

If set to controlled but not required the user can select items from the list which triggers the onValue event. This is expected and needed. However, if the user then emptied the Typeahead field so that there is no selection, the onValue event is NOT triggered. This is a critical bug as it does not allow for status changes on dependent widgets.

Package Version: @dojo/widgets 7.0.3

**Code**

The issue can be seen with Validation.tsx example from the Dojo widgets documentation. Just remove the required attribute from the widget, add alert("value=" + value ? value.label :" -"); to the existing onValue event.

**Expected behavior:**

The onValue event is triggered will value undefined (to set to empty the Typeahead wants undefined instead of null, so the event should use undefined as well).

**Actual behavior:**

The onValue event is not trigger when the user changes the Typeahead to be empty