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);
fori 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.