Here is a simple tip to enter null value in SQL Server Management Studio. It is a common case to enter null value into a cell. By default the cell values will be displayed with null if the table data is opened in SSMS. If we need to enter null value into a cell which has some data already into it we should explicitly type as NULL. Note that this should be in CAPS and if we give as null, SSMS assumes this null as a string value. A shortcut key for entering NULL value is ctrl+0 [ctrl key+zero].
Hope this helps