I wouldn't recommend to leave it blank. VM uses the suffix to distinguish between a product details page and category pages. If you leave it blank, you will most likely have some router errors. Since you said you updated from VM 1.x, you can change it to .html if you like to keep the file type at the end of the url. The file type is also written into the source code of your web page headers as 'DOCTYPE html. All search engine bots and browsers do look there to find the file type. Certain file types are forbidden of course, like for example .exe.
The -details at the end doesn't hurt your SEO, but if you don't like it, you can change it i.e. to -htm, -html, .php or something slightly more creative like -shop or -gift, if you disable the "Add Suffix to URL" in Joomla configuration. (Search engines don't need a suffix, but crawl them).
Google itself doesn't use .htm or .html either. And the only joomla.org page which still uses it, seems to be the homepage.