Interesting question. But at the least I will have to question the explanation ("If a subquery returns any null value the main query will not return any rows"). If you change the query to:

select * from dbo.Employees

where EmployeeID in (SELECT NullableColumn FROM NullOperation)

Then a single row is returned (1), even though the subquery still returns a null value. So I think a better explanation is that the IN operator returns NULL if the value is not found in the list, and the list contains NULL values.