Problem:
When handling a query like this:
  VALUES ('') UNION SELECT _utf16 0x0020 COLLATE utf16_bin;
Type_handler_string_result::Item_hybrid_func_fix_attributes()
tried to apply character set conversion Item_type_holder,
which causes a crash on DBUG_ASSERT(0) inside Item_type_holder::val_str().

Fix:
Overriding Item_type_holder's methods to avoid this, as follows:

  bool const_item() const { return false; }
  bool is_expensive() { return true; }