Error in the date field (with NULL value...).Thanks!

Subject: Importing data from Informix to PostgreSQL.
              Error in the date field (WITH NULL value)

Hello!
I'll try to explain my little problem. Well, I have this table

create table  mytable
   ( codice     char(16) not null,
     dt_inizio  date,
     dt_fine    date,
     tipo_operazione char(1),
     causa_operazione integer
   );

ok!

I find out that pgsql:

In my example '' is the NULL value exported from Informix... !  (an ASCII file)

                    INFORMIX                       PostgreSQL
char(16)               ''              ------>       blank
string         (I think it's   ok! )
char(1)                ''               ------>       blank
string         (I think it's ok)
integer                ''               ------>         0              (is
it an error? )
date                   ''                ------>      ERROR!  Bad date
external representation ''


 >> select * from mytable ;

    codice                     | dt_inizio       | dt_fine |
tipo_operazione | causa_operazione

  ABCEDEFFFFFFFFF |
2001-03-28  |             |                       |            0
  XXXYYYAAA23C957Y |
2001-03-28  |             |                      |            0


clinica=# insert into mytable values ( '','03/28/2001', '' , '' , ''  );
ERROR:  Bad date external representation ''                   ^^^

PostgreSQL doesn't want '' as an input of a date with NULL value:
it's necessary to use this kind of insert:

 >> insert into mytable values ( '','03/28/2001',null,'','');
                                                                 ^^^^^^

Now there is a new line in the table:

                                   |
2001-03-28  |             |                      |            0


My question:
How can I resolv my problem?  I have a big data file to import where
in the 2nd date field there is '' instead of  null ....

How can I "binds" PostgreSQL to consider '' as null ?

Many thanks for any suggestions!

CIAO!
MAURIZIO

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