DB2 to_nchar() functions modifies the behaviour of coulmn from NULL to NOT NULL

We are migrating the Sybase  database to DB2 database. As the data has DBCS characters, we are changing the char/varchar datatypes to nchar/nvarchar types. As part of this, the function TO_CHAR is changed to TO_NCHAR() assuming that the behaviour of both is same. But it is observed that while creating a table with TO_NCHAR() gives the column as NOT NULL whereas creating a table with TO_CHAR() gives the column as NULLABLE.

Source table:
 
CREATE TABLE tt_source
   (
   StartDate  TIMESTAMP NULL,
   enddate    TIMESTAMP NOT NULL
   )
 
Script used to create target table:
 
CREATE TABLE tt_target AS
   (
      SELECT
     
      TO_CHAR(Startdate,'mm/dd/yyyy') StartDate_char
     ,TO_CHAR(EndDate,'mm/dd/yyyy') EndDate_char
      ,TO_NCHAR(Startdate,'mm/dd/yyyy') StartDate_Nchar
     ,TO_NCHAR(EndDate,'mm/dd/yyyy') EndDate_Nchar
      FROM tt_source
   ) DEFINITION ONLY ;

The output is
Column      Type      Remarks
StartDate_char      Null      
EndDate_char      NOT NULL      
StartDate_nchar      NOT NULL      "This should have been NULLABLE,
                                                                  but to_nchar() changed to NOT NULL column"
EndDate_nchar      NOT NULL      


Is there a way to retain the nullability of the column while using TO_NCHAR() function.