`with_locales_no_fallback` does fallback

[robertbastian]

The following snippet panics because HelloWorldProvider doesn't have fallback data

DatagenDriver::new()
    .with_keys([icu_provider::hello_world::HelloWorldV1Marker::KEY])
    .with_locales_no_fallback([langid!("de")], Default::default())
    .export(
        &icu_provider::hello_world::HelloWorldProvider,
        icu_datagen::blob_exporter::BlobExporter::new_with_sink(Box::new(std::io::sink())),
    )
    .unwrap();