I'm unable to duplicate the problem you're having. I created a blank form with 11 numeric fields on it and if I leave one of them blank, it still takes the lowest number from the fields I placed values in. I named the fields n1-n11, here's the formcalc script I used.
this.rawValue = Min(n1,n2,n3,n4,n5,n6,n7,n8,n9,n10)
Could you post the script you're using?
Regards,
Dave