Letting a function work on NULL

I have a function that returns an integer for 100*year+month for a given
date. Currently it croaks if the date is NULL and I would like it to
return 0 (or even NULL).

Function:

create function ym (date) returns int4 as 'select
date_part(\'year\',$1)::int4*100+date_part(\'month\',$1)::int4 ' language
'sql';

I've tried:
create function ym (date) returns int4 as 'select
date_part(\'year\',$1)::int4*100+date_part(\'month\',$1)::int4 where $1 is
not null except select 0 where $1 is null' language 'sql';

(and variations with 0::int4, union instead of except, and a few desparate
others)

But all gives:

select ym(NULL);
ERROR: typeidTypeRelid: Invalid type - oid = 0

Is there any way to specify an exception on the input type?

Anton

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