Fix issue where close_spider raises ValueError when database is empty

[ngshiheng]

• If you run scrapy crawl foobar for the first time (e.g. on a fresh DB), ValueError(f"No existing product information was updated for {spider.name}.") will be raised
• This is inaccurate because it is expected that no existing product is found
• https://github.com/ngshiheng/burplist/blob/production/burplist/pipelines.py#L101