CS-BOLD-028 Informational 7.8: Inconsistent Input Validation by Zappers

[bingen]

Liquity uses separate Zapper contracts for WETH collateral and other LST collateral tokens. The contracts are inconsistent.
The WETHZapper enforces, in _adjustTrovePre(), that debt must decrease by a positive amount:

require(!_isDebtIncrease || _boldChange > 0, "WZ: Increase bold amount should not be zero");

However, GasCompZapper._adjustTrovePre() is missing that check, allowing calls with debtIncrease set to true and boldChange equal to zero:

if (_isCollIncrease || (!_isDebtIncrease && _boldChange > 0)) {
    _requireSenderIsOwnerOrAddManager(_troveId, owner);
}

If no add manager is assigned (set to address 0), anyone can call this function, but it has no effect on BorrowerOperations.adjustTrove(), as _adjustTrove() debt increases only have an effect if they are larger than 0.