PL/pgSQL how to pass null values to the functions?

I have plpgsql function that updates the table and may have (and also may
not have) null parameters:

create function f_test(int4, int4) returns int4 as
'
declare
v1 alias for $1;
v2 alias for $2;
v3 int4;
begin
raise notice ''v1 = %'', v1;
update _testtable set a = v1 where b = v2;
select into v3 a from _testtable where b = v2;
raise notice ''v3 = %'', v3;
return v3;
end;
' language 'plpgsql';

Column a in _testable can have null value. When I do ‘select f_test(1,1);’
then everything is working fine. The row is updated (v1 = v3 = 1).
But when I do ‘select f_test(null,1);’ nothing happens :-(. No errors
reported, but ‘select a from _testable where b=1;’ returns 1 (v1 = v3 =
<null>).
Where am I wrong?

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