Refactoring: Change Item_save_in_value() not to return SQL NULL

The return value for Type_handler::Item_save_in_value() is misleading. It currently returns:

• true if the value evaluated to SQL NULL
• false otherwise

Similar functions return true if some error happened. So the code looks confusing.

Let's change the return type from bool to void.

Let's also add a new method st_value::is_null(), to make SQL NULL tests simpler, as follows:

  bool is_null() const

  {

    return m_type == DYN_COL_NULL;

  }

so the callers, instead of testing the return value, can test value.is_null().

The caller code will change, for example, like this:

   st_value tmp;

-  if (!item->save_in_value(thd, &tmp))

+  item->save_in_value(thd, &tmp);

+  if (!tmp.is_null())