[EuiSuperSelect] Arrow keys don't modify focus when no valueOfSelected is provided

[brittanyjoiner15]

We noticed an issue where when we don't have an initial selection for our EuiSuperSelect element (Subscription field in this example), then the arrows don't work to navigate through the options. We have to click tab twice to be able to bring the dropdown options into focus and use arrows to navigate through them (or make a selection, and then we are able to click into the options and use arrows to navigate).

cc @i-a-n