Try it in other way, i.e. If value of column is not equal to 'Test' then some field.
Dont use else part and end the case statement.
See if it works as required.
If it is a text field, then it is straight forward I believe.
See below example.
The '' represents the NULL which you want.
The trick is to click on '+ Add' button without anything entered in the box before it, which will insert '' in the CASE statement
This below code is from a actual test report that I created now
CASE WHEN Assigned Group = 'Test' THEN '' ELSE 'some string' END
It can be achieved using case-when-then statement where conditional column value results can be replaced with literal strings '' (to display no data/empty)
e;g My Summary column value is 'test incident' which i want to set to blank/no data. Using calculated field, I will write case-when-then statement as below and use this calculated field field in report.
CASE WHEN Summary = 'test incident' THEN '' ELSE Summary END
Hope this helps !!