What is NULL does is that if the value of that field in a record is empty in your case it will assign a value of 0. Since 0 <> 1 there will not be any records that satisfy the above condition. So you will just get the field name.
The above query states that
Get me data in field one where value in field 2 = 1. Since field2 doesn't have any value, you are assigning 0. as 0 is not equal to one, there is no matching criteria.