of <parameter> IS NULL":
we must not only set Item::null_value in Item_param, but implement
Item_param::is_null() to work well with IS NULL/IS NOT NULL clauses.


mysql-test/r/ps.result:
  Test case for Bug#6297: test results fixed.
mysql-test/t/ps.test:
  A test case for Bug#6297 "prepared statement, wrong handling of 
  <parameter> IS NULL"
sql/item.h:
  A fix for Bug#6297: we must not only set null_value in Item_param, but
  also implement Item_param::is_null() to work well with  IS NULL/
  IS NOT NULL.
  Item::is_null() commented.