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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