Currently the circuit does not use an Exit Leaf at the Exit MerkleTree
when the Amount=0 & EffectiveAmount=0, but it does use an Exit Leaf when
the Amount>0 but EffectiveAmount=0 (for example when tx.Amount >
sender.Balance). This is a particularity of the approach of the circuit,
the idea will be in the future to update the circuit and when Amount>0
but EffectiveAmount=0, to not add the Exit in the Exits MerkleTree, but
for the moment the Go code is adapted to the circuit and when an Exit
with Amount>0 & EffectiveAmount=0, it will be added to the Leaf of the
Exit MerkleTree.