function returning NULL?

I'm trying to create a function that should return a date, and occasionally
NULL value. I can't see how to do that. I'm using pl/tcl.

CREATE FUNCTION "start" (text,text) RETURNS date AS '
regsub ^. $1 "$2" job
spi_exec "select start_date from jobs where job_code = ''$job''" {
if {[info exist start_date]} {
return $start_date
}
}
# return "2/2/2222" HERE I want NULL
return "2/2/2222"
'LANGUAGE 'pltcl';

That is to say I'd like to have a function that returns a date but under
some circumstancies it leave it blanck.

TIA
sandro
*:-)

--
Sandro Dentella *:-)
e-mail: sandro(at)ermit(dot)it sandro(dot)dentella(at)mi(dot)infn(dot)it