As defined by the SQL-92 Standard, when comparing two strings of differing widths, the narrower value is right-padded with spaces to make it is same width as the wider value. Therefore, all string values that consist entirely of spaces (including zero spaces) will be deemed to be equal e.g.

'' = ' ' IS TRUE
'' = '  ' IS TRUE
' ' = '  ' IS TRUE
'  ' = '      ' IS TRUE
etc

Therefore, this should work regardless of how many spaces make up the some_col value:

SELECT * 
  FROM T
 WHERE some_col IS NULL 
       OR some_col = ' ';

or more succinctly:

SELECT * 
  FROM T
 WHERE NULLIF(some_col, ' ') IS NULL;

You should list out all the columns as per JNK's comment.

WHERE c1 IS NULL OR c2 IS NULL OR c3 IS NULL

A somewhat less efficient approach that avoids this is below though.

;WITH xmlnamespaces('http://www.w3.org/2001/XMLSchema-instance' AS ns) 
SELECT * 
FROM   YourTable AS T1 
WHERE (
    SELECT T1.* 
    FOR XML PATH('row'), ELEMENTS XSINIL, TYPE
  ).exist('//*/@ns:nil') = 1 

(Based on this SO answer)

Solution 1 : Use IsNULL() Function, When below query return null value IsNULL function replace null value with 0 and if condition treated as False.

IF EXISTS (SELECT IsNULL(TU.Tagged,0) FROM TopicUser TU 
          WHERE TU.TopicId = @TopicId and TU.UserId = @UserId)
BEGIN  
END

Solution 2 : Use (IS NULL OR IS NOT NULL) Property.

IF EXISTS (SELECT TU.Tagged FROM TopicUser TU 
          WHERE TU.TopicId = @TopicId and TU.UserId = @UserId
          AND TU.Tagged IS NOT NULL)
BEGIN

END