I am not able to check it, but my thoughts are - mandatory field status is a problem.
It is possible to enter zero interest condition - just enter new condition starting from new date and leave percentage field empty or enter zero value ( zero value = empty field). But the problem is that percentage field is mandatory as you mentioned.
Possible solution: switch off mandatory status and create implementation of BADI to check if field is filled (for example Check entry of deal's data.) and if not - issue an error that there will be zero condition.