Vinay,
This is by design. I raised this with SAP and DI rules assume all fields are populated, and a NULL in any field will produce a failure in this scenario. I raised this idea to hopefully change the current design.
For now, all rules will need to cater for all fields potentially having a NULL or you introduce an NVL to convert from NULL in all parameters.
Feel free to vote for the idea and with any luck SAP will change the functionality at some point.
regards
Adrian