MDEV-26953 Assertion `!str || str != Ptr || !is_alloced()' failed in String::copy upon SELECT with sjis

Item::save_str_in_field() passes &Item::str_value as a parameter
to val_str().

Item_func::make_empty_result() also fills and returns str_value.

As a result, in the reported scenario in
Item_func::val_str_from_val_str_ascii()
both "str" and "res" pointed to Item::str_value,
which made the DBUG_ASSERT inside String::copy()
(preventing copying to itself) crash:

  if ((null_value= str->copy(res->ptr(), res->length(),
                             &my_charset_latin1, collation.collation,
                             &errors)))

Fix:
- Adding a String* parameter to make_empty_result()
- Passing the val_str() parameter to make_empty_string().