“NULL” is never equal to anything, even itself. Therefore comparisons using equality operators will always return False, even when the value actually IS NULL.
For that reason, comparison operators should never be used to make comparisons with NULL; IS NULL and IS NOT NULL should be used instead.
Bad example:
UPDATE books SET title = 'unknown' WHERE title = NULL -- Noncompliant
Good example:
UPDATE books SET title = 'unknown' WHERE title IS NULL