Preserve Nulls Option
If you want to preserve null values in a masked column then select the Preserve Nulls checkbox in the Masking view.
You can also access this setting using the popup menu (right-click) in the explorer tree and diagram.
* This setting shall be disabled for columns whose DBMS data type definition specifies that it is not nullable.
* This setting is not available on columns that are members of a Column Group. This is because a Column Group means that all columns are masked together per row and therefore a column cannot be masked individually which also means that a preserve property cannot apply to an individual column of a Column Group. Therefore, you would use a Preserve mask and specify its Where condition to define the rows on which every column of the Column Group should be preserved.