Fixed: Use Array.Empty and fix a few multiple enumerations

(cherry picked from commit 11d91faaada0e70910c832ce405ddeed52a24172)

src/NzbDrone.Core/Indexers/IndexerFactory.cs

@@ -189,7 +189,7 @@ namespace NzbDrone.Core.Indexers
 
         public override IEnumerable<IndexerDefinition> GetPresetDefinitions(IndexerDefinition providerDefinition)
         {
-            return new List<IndexerDefinition>();
+            return Array.Empty<IndexerDefinition>();
         }
 
         public override void SetProviderCharacteristics(IIndexer provider, IndexerDefinition definition)