Use Array.Empty and fix a few multiple enumerations

(cherry picked from commit 11d91faaada0e70910c832ce405ddeed52a24172)

src/NzbDrone.Core/Indexers/RssParser.cs

@@ -85,7 +85,7 @@ namespace NzbDrone.Core.Indexers
 
             if (!PostProcess(indexerResponse, items, releases))
             {
-                return new List<ReleaseInfo>();
+                return Array.Empty<ReleaseInfo>();
             }
 
             return releases;