urFragmentChanged does deal with null fragments

[bsutton]

The method Wizard.uriFragmentChanged doesn't deal with the fact that a fragment can be null.
The method needs to be changed to:

@Override
public void uriFragmentChanged(UriFragmentChangedEvent event) {
    if (isUriFragmentEnabled()) {
        String fragment = event.getUriFragment();
        if ((fragment == null || fragment.equals("")) && !steps.isEmpty()) {
            // empty fragment -> set the fragment of first step
            Page.getCurrent().setUriFragment(getId(steps.get(0)));
        } else {
            activateStep(fragment);
        }
    }
}