Fix metadata search crash when ComicVine or Douban return None on error

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

cps/metadata_provider/comicvine.py

@@ -57,7 +57,7 @@ class ComicVine(Metadata):
                 result.raise_for_status()
             except Exception as e:
                 log.warning(e)
-                return None
+                return []
             for result in result.json()["results"]:
                 match = self._parse_search_result(
                     result=result, generic_cover=generic_cover, locale=locale

cps/metadata_provider/douban.py

@@ -155,7 +155,7 @@ class Douban(Metadata):
             r.raise_for_status()
         except Exception as e:
             log.warning(e)
-            return None
+            return []
 
         match = MetaRecord(
             id=id,