Null Value in Table

I have a set of date fields in a column. The data type is kept as date only in Power BI. 

When the cells are blank then i need to display NULL. However when i am trying to createa  calculated column, it is not support since NULL can't exists in date datatype.

At the same time i cant change the date datatype to text since i need to maintain the format DD-MMM-YY.

Please help.