Improve handling of nulls and blobs in binary fields

Created by: nshahquinn

Previously, a null value in a binary-encoded MariaDB field would cause an AttributeError. These are rare in our infrastructure since MediaWiki generally uses empty strings instead.

I had assumed that MediaWiki did not use blob fields, but it turns out they are very common. In all the cases I found, the fields contain UTF-8 string data, so these are now decoded as well.