CAST(null as date)...

I tried to create a function to return the string 'null' (without quotes, of course...) if the input was a zero length string, so I could use it in casting arguments to another function such as:

select myfunction(cast(nullifzls($maybeemptyvar) as date), cast(....));

However I have this dilemma. The return type from the nullifzls function is text. Text blows up the cast. Is there any way to make this work, or should I do something else?

plantest=# select cast(null as date);
?column?
----------

(1 row)

plantest=# select nullifzls('');
nullifzls
---------------
null
(1 row)

plantest=# select cast(nullifzls('') as date);
ERROR: Bad date external representation 'null'

plantest=# select cast(cast('null' as text) as date);
ERROR: Bad date external representation 'null'

Thanks...

Ian