Empty properties in UploadI18N cause client side errors

[tulioag]

If you set an I18n object, but leave fields empty, client side exceptions are shown.

@Route
@PWA(name = "My Application", shortName = "My Application")
public class MainView extends VerticalLayout {

    public MainView() {
        Upload upload = new Upload();
        upload.setI18n(new UploadI18N());
        add(upload);
    }
}

[web-padawan]

Fixed by #1906 which enabled setting partial UploadI18n and prevented the error from being thrown. Closing.