Fix #1360806: allow book.smart_update to copy "null" (Unknown) author information to the destination if the destination author and author_sort values are null. There is a small performance hit if the source of the copy is null, which fortunately is rather rare. This change better supports non-null author_sort values associated with null author values.
Fix #1360806 [{author_sort} ignored (when saving and in plugboards) when book has no author](https://bugs.launchpad.net/calibre/+bug/1360806)

Merge branch 'master' of https://github.com/cbhaley/calibre