Fixed: ArgumentNullException importing against a newly added edition with no series links

- Fixes Sentry READARR-2CY

(cherry picked from commit 4eebd4f2050ca646869b657bb365e11b1eea4d0c)

src/NzbDrone.Core/MetadataSource/Goodreads/GoodreadsProxy.cs

@@ -869,7 +869,7 @@ namespace NzbDrone.Core.MetadataSource.Goodreads
                 }
             }
 
-            return null;
+            return new List<SeriesBookLink>();
         }
     }
 }