Both comicvine.py and douban.py return None instead of [] when an HTTP
error occurs during a metadata search. The consumer in search_metadata.py
iterates the result directly, which raises TypeError: 'NoneType' object
is not iterable and crashes the server on single-threaded deployments.
Fixes#3606
Change search API
Change to Return an empty list when an error occurs
Change the way to get tags
Fix series and publisher perse error
Rename a variable with the same name as the built-in