CASE WHEN IS NULL THEN ELSE.

SELECT T1.BIRTHEMP,
  T1.BIRTHEMPDEP,
  (
  CASE
    WHEN T2.BLESSNUM IS NULL
    THEN 0
    ELSE T2.BLESSNUM
  END) AS BLESSNUM
FROM
  (SELECT E.EMP_NAME AS BIRTHEMP,
    M.NAME           AS BIRTHEMPDEP,
    E.USER_ID
  FROM M_EMPLOYEE E
  LEFT JOIN M_ORGANIZATION M
  ON M.ORG_ID                   =E.DEPT_ID
  WHERE TO_CHAR(E.BIRTHDAY,'MM')=TO_CHAR(SYSDATE,'MM')
  )T1
LEFT JOIN
  (SELECT TO_USER,
    COUNT(*) AS blessnum
  FROM T_ECM_BIRTH_BLESS
  LEFT JOIN M_EMPLOYEE M
  ON M.USER_ID=TO_USER
  GROUP BY TO_USER
  )T2 ON T1.USER_ID=T2.TO_USER