Who cares what it is? Setting to an empty byte array is fine, IMO. The payment protocol is already rolling out. It's implemented in several wallets, BitPay implements it, Coinbase is implementing it, etc.

-100000 for changing such a basic thing at this point. It'd cause chaos for the early adopters, punishing them instead of rewarding them. It'd seriously hurt adoption of the payment protocol when it's at its most vulnerable. We should mark BIP 70 as accepted and be done with it.