Normally when comparing a NULL with a NULL for equality you’ll get a NULL result.
But that can be overridden with the transform_null_equals configuration parameter at the database or session level!
Example:
yugabyte=# show transform_null_equals;
transform_null_equals
-----------------------
off
(1 row)
yugabyte=# SELECT NULL = NULL;
?column?
----------
(1 row)
yugabyte=# set transform_null_equals = on;
SET
yugabyte=# SELECT NULL = NULL;
?column?
----------
t
(1 row)
Have Fun!