Try this
SELECT T0.[USER_CODE], T0.[U_NAME], Case when Isnull((Select Min(T1.updatedate) from Ausr T1 where T0.USERID = T1.userSign), '') <> '' then (Select Min(T1.updatedate) from Ausr T1 where T0.USERID = T1.userSign) else T0.[updateDate] end 'Creation date', T0.[lastLogin], T0.[Locked] FROM [dbo].[OUSR] T0