If you were forecasting where adv spends = 0 then as you suggested you would specify the future values as zero. Care should be taken when estimating coefficients that unusual values or level shifts be detected and incorporated otherwise you run the serious risk of having coefficients that are incorrect.

If you were forecasting where adv spends = 0 then as you suggested you would specify the future values as zero. Care should be taken when estimating coefficients that unusual values or level shifts be detected and remedies be incorporated otherwise you run the serious risk of having coefficients that are incorrect.