case expression has no data type (all branches are NULL)

  Explanation:  The result data  type  of  a  CASE  expression  is
  calculated  from  each of the result value expressions following
  the THEN clause.  This CASE expression only has NULL  which  has
  no  data type attributes.  (Note that a missing ELSE clause also
  defaults to NULL.)

  User Action:  If you want to use NULL,  then  use  CAST(NULL  AS
  some-dtype) to apply a data type to the NULL specification.