Removed the need to pad the state tree

[weijiekoh]

Initially, the contract created the state tree with a zero value equal to the nothing-up-my-sleeve value. This created the problem of having to pad the state tree with blank empty leaves, as the batch update state tree circuit requires a complete batch of state leaves to compute each state root. Without the preimage of the nothing-up-my-sleeve value, the only way to supply valid inputs to the batch UST circuit was to pad the state tree, both on- and off-chain, with empty leaves.

This PR solves this problem by initialising the state tree with the hash of a blank state leaf as the zero value.