Corrected results after the fix for bug #12791.
func_test.result, func_test.test:
  Added test cases for bug #12791.
item_func.h, item_func.cc:
  Fixed bug #12791.
  Made LEAST/GREATES fully Oracle compliant.
  LEAST/GREATEST did not return NULL if only some
  arguments were NULLs. This did not comply with Oracle.