Forum OpenACS Development: Response to PgBindCmd turns the empty string into NULL
So there's symmetry here.
Dave ... the db_null idea's a noble failure, unfortunately, because it doesn't work with either real bindvars or bindvar emulation. Or rather the only value that will work is the empty string, for any RDBMS we use, so it's pointless.
Stripping it out and all references to it is one of those very low-priority things on my personal list for us to clean up someday.