TroveManager.getCurrentICR(borrower, price).
OptionaltroveTroveManager.getTroveOwnersCount(), or undefined when it was not read (MK-074).
_liquidate returns an empty result without liquidating when TroveOwners.length <= 1
(TroveManager.sol:1058-1060), so batchLiquidateTroves accumulates nothing and reverts
at :690-693 with "TroveManager: nothing to liquidate". The last Trove in a system is
never liquidatable, whatever its ICR. Note this is the count ALONE: unlike the close
path's _requireMoreThanOneTroveInSystem (:1488-1496), the liquidation path does not
also consult sortedTroves.getSize().
undefined is "not asked", not "there is more than one", so the condition is skipped.
Inputs to isTroveLiquidatable: a live ICR and, optionally, the system's Trove count.