Avoiding zero in bach muls

[codygunton]

As in the case of https://github.com/AztecProtocol/aztec-packages/pull/5174, we sometimes encounter issues relating to our inability to handle scalar multiplication of EC points in the case where the scalar is 0. I believe: this could be fixed by simply checking whether z==0 before https://github.com/AztecProtocol/aztec-packages/blob/394a0e06928946c1c9eea1bdfec39269cb2d601a/barretenberg/cpp/src/barretenberg/ecc/groups/element_impl.hpp#L68, but we avoid this because of the cost of the check. Someone should determine whether this is really a concern, and if it is, we should identify some hot code paths and use a more robust method outside of those cases.