Improve on the return values of `innerTournamentWinner` call

[stephenctw]

The call should be able to reflect the state of the tournament correctly, being one of the three below:

1. Tournament not finished.
2. Tournament finished, no one won (parent can delete parent match).
3. Tournament finished, here's the winner.

@return (bool, Tree.Node, Tree.Node)

The return values could be:

1. (false, zeroNode, zeroNode)
2. (true, zeroNode, zeroNode)
3. (true, winnerParentCommitment, winnerCommitment)

Currently we handled (1) and (3), missing (2). @GCdePaula do you think the return values of (2) above makes sense?