TZIP-7: Should getAllowance return 0 for non-existing accounts?

Two edge-cases are unspecified in TZIP-7 regarding getAllowance(from, to):

• what should happen when the address from is not registered? Note that getBalance(addr) returns 0 if addr is not registered, it makes sense to me to be coherent.
• what should happen if from is registered, but has no allowance for to? Here it seems obvious that the result should be 0, but it bares making explicit.

It would also make sense to see how existing FA1.2 implementations handle these cases. Probably, they either throw and error or return 0.