BannerImage: Fix `empty` property definition

The bannerImage id used before refers to an id assigned to this component when instantiated by client code (in other/public Kirigami types). So while it kinda worked, is was obviously error-prone and just wrong.

Next, source.length and titleIcon.length were simply wrong. Source is a QUrl type which (as any other QVariant-wrapped type in QV4 runtime) has no length property; and similarly titleIcon is an even more generic QVariant which can be anything that Kirigami.Icon supports: even Pixmap!

CC @sporknife