Avoid setting nil amount if tax calculation is NaN.

Not sure that this is the correct solution, but it does solve the problem of nil adjustments getting created in Spree.

This problem was first discovered when removing the last item from the cart using the 'X' delete. Feels like this is an indication of some other problem upstream.

This addresses issue reported on spree/spree: https://github.com/spree/spree/issues/3348