zcash_client_backend: Change `propose_shielding` to return `Option<Proposal>`

[str4d]

If there are no transparent funds to be shielded, currently an error will be returned (either because the balance to be shielded is below the shielding threshold, or because an empty proposal cannot be constructed). We should instead return None in this case, as an expected non-failure case: if there is nothing to shield, don't make a proposal to create shielding transactions.