You can change how Null and Blank fields are displayed in a Report.
Head to the Business Intelligence menu.
Find the report you wish to amend.
Then, click the Edit icon at the top right of the screen.
In the edit view, select the element you want to alter.
In the Inspector go to the Special Data section.
Here you can decide to Show Null as something else i.e. "Unknown" "TBA" or remove [NULL] which will leave the cell empty.
Note: If a report has Null and/or Blank fields you may want to display these fields as a 0 or a dash (-) as an example or even just empty.