contracts: Passing zero as deposit_limit to call_v2 correctly limits the deposit to nothing (#3392)

Only affects unstable `v2` versions of `call` and `instantiate`. Also
only when `0` was passed as `desposit_limit`. Old behaviour was allowing
all the remaining balance. New correct behaviour is to not allow any
deposit. Otherwise both passing the `SENTINEL` and encoding `0` would
mean the same thing.