Query result had 0 where it should have had NULLs:
- Make Item_window_func::val* functions honor NULL-handling conventions:
  1. set null_value to indicate whether we've returned a NULL value
  2. val_str and val_decimal should return NULL pointer when they're
     returning SQL NULL value.

Fix assertion failure when sending results to network.
- The assert was due to window func returing SQL NULL despite having
  maybe_null=false
- Fixed by settting Item_window_func::maybe_null correctly in fix_fields