WFA::getSomeWeight: make more robust by returning the weight on any state

getSomeWeight used to:
1. Look for a transition and return the weight on it
2. If no transitions, return the weight on the initial state

Now it:
1. Looks for a transition and return the weight on it
2. If no transitions, return the weight on an arbitrary state

The upshot means that if you have a WFA with no transitions, you can call 'addFinalState' and some other stuff before calling 'setInitialState'.