Refactor spider_db_mbase_util::open_item_func()

spider_db_mbase_util::open_item_func() is a so-called monster function. It is difficult to maintain while it is expected that we need to modify it when a new SQL function or a new func_type is added.

• spider_db_mbase_util::open_item_func(Item_func *item_func, ...) breaks almost every time new Item_func::Functyp} is added (e.g., MDEV-24760, MDEV-24517). A possible solution would be removeing the default case and enable -Wswitch.
• Split the function into two different functions: one handles the case of str != NULL and the other handles the case of str == NULL.

NOTE: You don't need to make everything ideal at once, and it's impossible to do that with complex middleware like databases. Refactor only to the extent that you are sure it is safe to do so without additional unit tests.