In commit “[RPC] [wallet] allow getbalance to use min_conf and watch_only”
I think the safest thing here would be to raise an error whenever accounts are deprecated and account is non-null, because previously getbalance(null), getbalance("*"), and getbalance("") each did different things, and it doesn’t seem good to silently ignore a requested behavior. Another slightly less safe but maybe acceptable option would be to change "" to "*" on this line, since getbalance("*") and getbalance(null) at least were more similar than getbalance("*") and getbalance("")
Previously, and currently with the deprecated accounts feature enabled:
getbalance(null) would return total balance
getbalance("*") would return total balance with legacy accounting method
getbalance("account") would return balance associated with a named account
getbalance("") would return leftover balance not associated with any named accounts