我有以下案例陈述；

case when (
    CASE 
    WHEN datediff(year,view_3.[PERSON BIRTH DATE], @ReportPeriodStartDate)  >= 19 AND view_3.[NI NUMBER] IS NOT NULL 
        THEN '' ELSE view_3.[NI NUMBER]  END ) = 0 then '' else view_3.[NI NUMBER] END AS [NI NUMBER],

我想把它还回去

Birthddate  NI Number   

14/04/2012  9000
06/05/2020  2000
01/01/2001
22/12/1998

但我得到的结果是；

Birthddate  NI Number   

14/04/2012  9000
06/05/2020  2000
01/01/2001  0
22/12/1998  0

我不要零，我要空字符串。