Null vs. Empty String in Postgres 8.3.8

Can someone provide a link to the Postgres 8.x documentation that provides information about Null vs. Empty String?
Here is the situation:
Currently,  the source code performs the following SQL statement :
UPDATE user_group set subversion_flags='' WHERE user_id='5818' AND group_id='438';
(This SQL statement worked in a very old version of Postgres)
and pgsql 8.3.8 returned ERROR:  invalid input syntax for integer: ""
The table
 \d user_group
                                     Table "public.user_group"
      Column      |     Type      |                            Modifiers
------------------+---------------+--------------------------------------------
---------------------
 user_group_id    | integer       | not null default nextval(('user_group_pk_se
q'::text)::regclass)
 user_id          | integer       | not null default 0
 group_id         | integer       | not null default 0
 subversion_flags | integer       | not null default 0
.
.

I know probably the best way is to the put check in the application level (making sure that subversion_flags has a
value)before the actual update SQL call; however, I really just want to port the code to work with Postgres 8.3.8
avoidingany code changes if that's possible.   Is there something that I can do at the database level (like alter the
table)so that I can still use the same SQL statement in the application level listed above and it wouldn't return an
error?Any ideas? 

Thanks in advance
Mary