What are the values will store Interim Successor in back end tables

Interim Successor is a field in the Succession Plan. This field should be checked or unchecked. The value is stored in the EMERGENCY_SUCCESSOR field and should be stored as ‘Y’ or ‘N’, but it is storing blank values as well. Can someone explain why it is storing blank values ?

Bellow attached querry.

SELECT distinct EMERGENCY_SUCCESSOR
FROM HRM_PLAN_CANDIDATES