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 avoiding any 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