Allow the Null Fields

I have a little question concerned one script to execute on ORACLE.

How to modify a column to accept null fields by putting them between " "? the column that I want to modify is the column " CONTACT ".

DROP  TABLE ITIN38.EXT_S_APPEL;
 CREATE TABLE ITIN38.EXT_S_APPEL 
   (    ID NUMBER NOT NULL ENABLE, 
    DATE_APPEL VARCHAR2(29 BYTE) COLLATE USING_NLS_COMP NOT NULL ENABLE, 
    SENS_APPEL_CODE NUMBER(*,0) NOT NULL ENABLE, 
    TYPE_EVENEMENT_CODE NUMBER(*,0) NOT NULL ENABLE, 
    NUMERO VARCHAR2(20 BYTE) COLLATE USING_NLS_COMP, 
    CONTACT VARCHAR2(100 BYTE) DEFAULT ON NULL '' COLLATE USING_NLS_COMP, 
    LIGNE_ID NUMBER(*,0), 
    COMMENTAIRE CLOB COLLATE USING_NLS_COMP, 
    DATE_MAJ VARCHAR2(29 BYTE) COLLATE USING_NLS_COMP NOT NULL ENABLE, 
    ETAT_APPEL_CODE NUMBER(*,0) NOT NULL ENABLE, 
    EVENEMENT_RATT_ID NUMBER
   )  DEFAULT COLLATION USING_NLS_COMP 
   ORGANIZATION EXTERNAL 
    ( TYPE ORACLE_LOADER
      DEFAULT DIRECTORY TRANSPORTS38_PCTC
      ACCESS PARAMETERS
      ( RECORDS DELIMITED BY newline
    SKIP 1
    CHARACTERSET UTF8
    BADFILE 's_appel.bad'
    LOGFILE 's_appel.log'
    FIELDS TERMINATED BY ';'
    OPTIONALLY ENCLOSED BY '"'
    (
        ID CHAR(100),
    DATE_APPEL CHAR(100),
    SENS_APPEL_CODE CHAR(100),
    TYPE_EVENEMENT_CODE CHAR(100),
    NUMERO CHAR(100),
    CONTACT CHAR(100),
    LIGNE_ID CHAR(100),
    COMMENTAIRE CHAR(10000),
    DATE_MAJ CHAR(100),
    ETAT_APPEL_CODE CHAR(100),
    EVENEMENT_RATT_ID CHAR(100)
    )
        )
      LOCATION
       ( 's_appel.csv'
       )
    )
   REJECT LIMIT 1000 ;

Can you please suggest me a solution for this script at the column level? I tried with this one but doesn't work :/

CONTACT VARCHAR2(100 BYTE) DEFAULT ON NULL '' COLLATE USING_NLS_COMP,