Dealing With Null On Database Lavel

Hi
post a sample code
create table testNull(
a int not Null,
b varchar(5),
c varchar(5) )

insert into testNull (a,b,c) values(1,'Alex','test')
insert into testNull (a,b) values(2,'Alex2')



1. select * from testNull
-- return 2 rows
2. select * from testNull
where
a <> 3
and
b <> 'C1'
and
c <>'C2'
-- return ONLY 1 ROW !!!
3. select * from testNull
where
a <> 3
and
b <> 'C1'
and
isNull(c,'') <>'C2'

query 2 will retun only 1 row, because value of column c is Null

Question
Is any setting could be changed on db or server level to prevent errors with missing row in 2-nd query , or I have use isNull operator for every column acepting Null as value ?