[Maria-developers] String and decimal results in case of error.

Hi! There was that in case of errors Item::val_str() and Item::val_decimal() can return NULL (0) but I found some parts of the code do not allow it, for example Item_sum_sum::add(). Should we revise val_* methods so that always retun something (for example subquery EXISTS was changed (and even we have subselect_debug.test about it), but single row subquery was not) or change places where val_str() and val_decimal() that it will allow NULL (0).