author list fails with empty filter

This is also happening on the keyword list. Here is a patch that fixes the issue there as well.

I have tested the patch. It is working as accepted. +1 for RTBC.

Thanks for the patch.

It would be more maintainable if there was a variable declared at the top for the $args param to db_query(); it would start as an empty array. At the point that the filter is added to $where, add the ':filter', etc. to $args.