I'm not saying an empty vote enters anything into the database. In fact, I'm sure it doesn't. I'm saying, rather, that an explicitly selected "+=0" vote would be sent by the browser for the site to process (into a noop) while not having either "++" or "--" selected does not result in anything being sent.

For example, on a page with ten nodes, you vote on one of them. With the default setting, the site receives one selection—for the one node you voted on. With "+=0" present (and selected in nine places), it gets selections for all ten nodes, because that's what the browser sends. I haven't seen the site's code, but I'd be surprised if it doesn't then discard all the "+=0" votes.