Hi @karynf,
You can use the "Taken into account if" setting inside the Update data activity.

All you need is to put a condition that checks or NULL values and returns either true or false.
You will have to use this at each column that you suspect will be holding NULL values.
Please be aware that it will cause an overhead in processing of updates and hence take into account that extra time when scheduling your import workflows.
Regards,
Vipul