Promotion actions do not need to calculate amount before creating adjustments

Spree::Adjustment re-calculates all adjustments when one is created, so its redundant to do the calculation twice.

Original behaviour was designed to check if the amount was 0 and then not add adjustment if so. However it is unclear why this behaviour is desirable as adjustments that previously calculate an amount of 0 may not do so later.
Use `&&` instead of `and`.