Common Syndication Parser sometimes creates php warning when feed item title is empty

Issue Summary

Under certain circumstances, the common syndication parser generates log warnings when it encounters a feed item with an empty title in a feed.

To reproduce:
1. Create a new feed with the URL:
http://artwash.blogspot.com/feeds/posts/default?alt=rss
2. Check /admin/reports/dblog.
3. You should see some php warnings "..Undefined offset...".

This seems to be a slow-moving feed, so that recipe should work for several months, until the items with empty titles drop off the end of the feed.

The problem is with the _parser_common_syndication_title function and just requires a very simple one-line fix.

A patch is attached. It's my first time making a patch file, so let me know if I've got the directories or anything else wrong!