Adding test case.
item_strfunc.cc:
  bug#11728 string function LEFT, strange undocumented behaviour
  Fixing LEFT and RIGHT return NULL if the second
  argument is NULL.


sql/item_strfunc.cc:
  bug#11728 string function LEFT, strange undocumented behaviour
  Fixing LEFT and RIGHT return NULL if the second
  argument is NULL.
mysql-test/t/func_str.test:
  Adding test case.
mysql-test/r/func_str.result:
  Adding test case.