load_file() string-function should return NULL rather than throw an error if
the file doesn't exist, as per the manual.


mysql-test/t/outfile.test:
  expect NULL rather than error if file given to load_file() doesn't exist
mysql-test/t/func_str.test:
  show that load_file() will return NULL rather than throw an error
  if file doesn't exist
mysql-test/r/outfile.result:
  expect NULL rather than error if file given to load_file() doesn't exist
mysql-test/r/func_str.result:
  expect NULL rather than error if file given to load_file() doesn't exist
sql/item_strfunc.cc:
  load_file() should return NULL as per the docs if file not found,
  rather than throw an error