BUG #1904: using distinct with null col causes error

The following bug has been logged online:

Bug reference:      1904
Logged by:          Brian Cox
Email address:      bcox@timestock.com
PostgreSQL version: 8.0.3
Operating system:   Linux (Fedora Core 2)
Description:        using distinct with null col causes error
Details:

any_db=> create view view1 as select null as col1 from any_table;
CREATE VIEW
any_db=> select distinct col1 from view1;
ERROR:  failed to find conversion function from "unknown" to text