Hi Shalini,
If you have only [Measure] key figure in table , by default Null values get eliminated from table.
If you have any other key figures in table then use below
Create a measure variable with [Measure] object as
[Measure1]=[Measure]+0 .
=sum([Measure1]where ([Measure1]>0))/count([Measure1] where ([Measure1]>0))