Item_field::print method does not take into
account fields whose values may be null.
The fix is to print 'NULL' if field value is null.


mysql-test/r/explain.result:
  test case
mysql-test/r/func_str.result:
  result fix
mysql-test/r/having.result:
  result fix
mysql-test/r/select.result:
  result fix
mysql-test/r/subselect.result:
  result fix
mysql-test/r/union.result:
  result fix
mysql-test/t/explain.test:
  test case
sql/item.cc:
  print 'NULL' if field value is null.