help with TCL function

Hello everybody,

Someone helped me earlier with this TCL trigger function:

create or replace function tlow() returns trigger as '
  set NEW($1) [string tolower $NEW($1)]
  return [array get NEW]'
language 'pltcl';

I use it to force lowercase of values inserted in the db. There is one 
problem though, if the parameter is absent, the function will give an 
error message. I would like it to rather check for null before tolower 
is applicated. In semicode:

if ($1 <> NULL) {
	set NEW($1) [string tolower $NEW($1)]
}
return [array get NEW]'

but I have no idea how to do this in TCL. Can somebody please tell me? 
Thanks a lot!