Don't assign emtpy guids which may possibly clobber valid guids.

Thanks so much for speedparser, it blows the socks off feedparser. Doesn't seem like speedparser is maintained anymore, but here's a fix nonetheless.

Came across a feed (http://feeds2.feedburner.com/mydealz) that tries to assign the guid for each entry twice, once with a valid guid, and once with a value of None, in that order - when this happens, the second assignment would clobber the first.

Without fully diving in, this change simply prevents guids with a value of None from being assigned, which fixes the issue.