aggregator_feed.image field should allow NULL values (and update to UTF-8)

Under MySQL 5, I'm getting an error when creating a new feed node. It the INSERT query isn't specifying a default value for the image field and the table definition doesn't provide one (because the field type doesn't allow it).

So, the two fixes are to either specify a default value in the INSERT query or remove the NOT NULL restriction. The attached patch removes the restriction.

Also, the patch also updates the Aggregator2 tables to UTF-8. If you'd rather this be submitted as a separate issue please let me know.