How to avoid nulls while writing string for dynamic query

Dear Friends,

Postgres 7.3.4 on RH Linux 7.2.

I wanted to write a dynamic query for insert statement.

create table test(c1 int, c2 varchar)

insert into test(c1, c2) values (1,'Hai1');
insert into test(c1, c2) values (NULL,'Hai2');

so I wrote a function called test_fn()

DECLARE
sqlstr VARCHAR(100);
rec RECORD;
BEGIN
FOR rec IN SELECT * FROM test
LOOP
sqlstr := 'insert into test(c1, c2) values (' ||rec.c1||','
||'\''||rec.c2||'\')';
RAISE NOTICE '%',sqlstr;
execute sqlstr;
END LOOP;
RETURN 'DONE';
END;

NOTICE: insert into test(c1, c2) values (1,'Hai1')
NOTICE: <NULL>

So i have created a null function.

sqlstr := 'insert into test(c1, c2) values (' ||ISNULL(rec.c1,'')||','
||'\''||rec.c2||'\')';

Now I got results as
NOTICE: insert into test(c1, c2) values (1,'Hai1')
NOTICE: insert into test(c1, c2) values (,'Hai2')
WARNING: Error occurred while executing PL/pgSQL function test_fn
WARNING: line 11 at execute statement

ERROR: parser: parse error at or near "," at character 34

The error is because of no value for column c1. If the column c1 is a string I might have replace it with empty string. I don't want to substitute with '0' which could work.

sqlstr := 'insert into test(c1, c2) values (' ||ISNULL(rec.c1,'0')||','
||'\''||rec.c2||'\')';

NOTICE: insert into test(c1, c2) values (1,'Hai1')
NOTICE: insert into test(c1, c2) values (0,'Hai2')

Total query runtime: 47 ms.
Data retrieval runtime: 0 ms.
1 rows retrieved.

How can I do that. Please advise me.

Thanks
Kumar