If you are searching for values that are NULL and non-NULL in the same statement, the search condition for the NULLs must appear separately.
For example, to select the names of all employees without the job title of 'Manager' or 'Vice Pres', plus the names of all employees with a null in the JobTitle column, you must enter the statement as follows:
SELECT Name, JobTitle
FROM Employee
WHERE (JobTitle NOT IN ('Manager' OR 'Vice Pres'))
OR (JobTitle IS NULL) ;