The problem was that when we have single row subquery with no rows
Item_cache(es) which represent result row was not null and being
requested via element_index() returned random value.

The fix is setting all Item_cache(es) in NULL before executing the
query (reset() method) which guaranty NULL value of whole query
or its elements requested in any way if no rows was found.

set_null() method was added to Item_cache to guaranty correct NULL
value in case of reseting the cache.