DATE(NULL)

NULL -- для выражения типа DATE

DATE('NULL')

CREATE TABLE tst_null_date(dt1 DATE, dt2 DATE, dt3 DATE);

CREATE VIEW v_tst_null_date(
  date_null
) AS SELECT 
         CASE 
           WHEN dt1 IS NOT NULL THEN
             dt1
         ELSE 
           CASE
             WHEN dt2 IS NULL THEN
               DATE(NULL)
           ELSE
             dt3
           END
         END
       FROM tst_null_date;

CREATE VIEW v_tst_null_date2(
  date_null
) AS SELECT 
         CASE 
           WHEN dt1 IS NOT NULL THEN
             dt1
         ELSE 
           CASE
             WHEN dt2 IS NULL THEN
               NULL
           ELSE
             dt3
           END
         END
       FROM tst_null_date;

-- DROP TABLE tst_null_date;
-- DROP VIEW v_tst_null_date;
-- DROP VIEW v_tst_null_date2;