There are 2 ways to interpret an Exit transaction with Amount=0:
`A`. Not adding a new Leaf to the ExitTree, so not modifying the Exit
MerkleRoot
`B`. Adding a new Leaf to the ExitTree, with Balance=0, which modifies
the Exit MerkleRoot

Currently the [Circuits](https://github.com/hermeznetwork/circuits) are
doing approach `A`, and the
[hermez-node](https://github.com/hermeznetwork/hermez-node) is doing the
approach `B`. The idea of this commit, is to use approach `A` also in
the hermez-node.