Don't use an Optional in `ItemFrame#getItemRotation`

Currently, `ItemFrame#getItemRotation` returns an `Optional`, as an item isn't always present in the `ItemFrame`. However, the rotation value is still meaningful without an item present.

In Vanilla, removing an item from an item frame preserves its rotation. When any item is placed back into the item frame, it will have the rotation of the last item.

Given that this behavior exists in Vanilla, it should be available to plugins through the API.