Fallback style works differently in Qt6

I tried to enable a custom style in an application, but learned that there's a stark difference in how fallback styles work between Qt5 and Qt6. I wonder if this is intentional, or a bug that should be fixed.

Non-existent styles do not use the fallback theme in Qt6, but instead emit an error and the engine stops like so:

qrc:/main.qml: module "foobar" is not installed

In Qt5, the fallback style is used like I would expect. We can work around this by checking the import path, but this gets complicated as most of the QQuickStyle API is gutted (like availableStyles). I'd rather not have to do that at all