Spider parse yield nothing

[flavioheleno]

Hey, first of all, nice work on this lib! Super easy to migrate from scrapy! :-D

I was wondering how one could handle SpiderInterface::parse() if, for example, what I'm looking to scrape from a page could not be found and I'm not going to crawl it.

I'm looking for a yield $this->empty(); - or something in that direction -, in line with the current yield $this->request(); and yield $this->item();.