validate empty cache file by filesize

[mvenghaus]

When the cache is enabled and the getRows method throws an exception, an empty database file is created. This file passes the exists and mtime checks and results in an error when the file is used the next time. You have do manually delete this file to fix it.

[inmanturbo]

Nice! This is something I run into often. Particular if there is a timeout when the cache is being created the first time. I time myself having to "warm up" an app that I maintain, to ensure the cache file filled successfully and the sushi model is in an operable state. I tried to do it in the queue, it doesn't always workout. If someone tries visit the page at just the wrong moment when the cache is rebuilding it gets stuck.