When storing a NULL to a TIMESTAMP NOT NULL DEFAULT ...,
NULL returned from some functions threw a 'cannot be NULL error.'

NULL-returns now correctly result in the timestamp-field being
assigned its default value.

mysql-test/r/type_timestamp.result:
  Show that for TIMESTAMP NOT NULL DEFAULT, NULL function returns
  set default now.
mysql-test/t/type_timestamp.test:
  Show that for TIMESTAMP NOT NULL DEFAULT, NULL function returns
  set default now.
sql/item.cc:
  When storing a returned NULL (from a time- or date-related
  function), make sure to apply NULL-means-DEFAULT magic where
  applicable.