character_set_results is nullable, but value_ptr returns string "NULL"

set_var.cc:
  Create Item_null instead of Item_string for NULL values

variables.result, variables.test:
  adding test case


sql/set_var.cc:
  Bug#12363
  character_set_results is nullable, but value_ptr returns string "NULL"
  Create Item_null instead of Item_string for NULL values
mysql-test/t/variables.test:
  fixing tests accordingly
mysql-test/r/variables.result:
  fixing tests accordingly