Problems with money fields.

Hello,

I was wondering if someone could shed some light on money fileds for me.  I
am having a problem in that a money field plus a null money field equals
null.  Try this:

create table test(total money);
insert into test values('');
insert into test valuse(NULL);

select * from test;
total
-----
$0.00    
(2 rows)

select '1'::money + total from test;
?column?
--------
$1.00          
(2 rows)

Why is the second column blank?  This is really throwing my calculations
because there are times when $0.00 is different from no value at all.
Also, how can I get a query of all rows where the money value is NULL?
This fails:

select * from test where total = NULL;
ERROR:  parser: parse error at or near "null"

and this gives me the rows with $0.00:

select * from test where total = '';  
total
-----
$0.00
(1 row)

If I can't query for a money field with a NULL value, why can I insert one?

Thank you,
Matthew Hagerty