Line items are currently calculated by a derived value of 

math:roundAmount(/fixedAmount - /details/discount + /quantity * /unitAmount)"

would it not be more appropriate to use:

math:roundAmount(number(boolean(/quantity))*/fixedAmount - /details/discount + /quantity * /unitAmount)

my xpath makes any value greater than 0 return a 1 for 

number(boolean(/quantity))

and a 0 if it is 0.

effectively this means that the fixed price is 0 if the qty is 0 rather than the current which sets the fixed price despite qty being 0.

Of course - I am not sure - but archetype changes like this might affect past finalized transactions - if it recalculates the value on the fly so perhaps it cant be changed.

Ben