Disable checkboxes on empty select + don't uncheck (bug 887189)

This disables the checkboxes if the price tier is an empty value e.g. set to the "Please select a price" option.

Also this changes the logic to no longer uncheck the checkboxes when disabling as it's unnecessary and it's better to keep it checked so it's picked up should a price tier change result in the same checkbox being newly enabled.