Generally, displaying nothing when the value is "null" is the desired functionality. Since the majority of the common users of the application might not completely comprehend when the null text is shown. It would be more talkative if the cell is left empty.
In other words, the described behavior is not a bug, rather, it is an intentional functionality. What I can recommend if you would like to show the null text is to utilize the template option of the grid's column: