maybe a bug in plpgsql, nulls and empty strings are not the same

When trying write a function in plpgsql I'm getting behavior that
probably isn't the corect one.

in the function bellow:

-----
-- split the given key
create function dad_char_key_split(
varchar, -- char_key
integer, -- subkey_len
char -- separator
) returns varchar as '
declare
p_char_key alias for $1;
p_subkey_len alias for $2;
p_separator alias for $3;
v_key_len integer;
v_from integer;
v_result varchar;
v_sep char;
begin
v_result := '''';
v_sep := '''';
v_from := 1;
v_key_len := char_length(p_char_key);
for i in 1..(v_key_len/p_subkey_len) loop
v_result := v_result || v_sep ||
substr(p_char_key,v_from,p_subkey_len);
v_sep := p_separator;
v_from := v_from + p_subkey_len;
end loop;
return v_result;
end;' language 'plpgsql';
----

if I try this:

select dad_char_key_split('00kjoi',2,',');

I get this result:

",kj,oi"

And when I change the initialization of the variables "v_sep" and
"v_result" from empty strings to a space ('' '' istead of '''') I get
the expected result:

"00,kj,oi"

It seems that plpgsql treats empty strings as null so when concatenating
with a empty string we get null instead of some value.