Prevent mapping null metadata responses

Fixes #2971

src/NzbDrone.Core/MetadataSource/BookInfo/BookInfoProxy.cs

@@ -507,11 +507,15 @@ namespace NzbDrone.Core.MetadataSource.BookInfo
 
         private List<Book> MapBulkBook(BulkBookResource resource)
         {
-            var authors = resource.Authors.Select(MapAuthorMetadata).ToDictionary(x => x.ForeignAuthorId, x => x);
+            var books = new List<Book>();
 
-            var series = resource.Series.Select(MapSeries).ToList();
+            if (resource == null)
+            {
+                return books;
+            }
 
-            var books = new List<Book>();
+            var authors = resource.Authors.Select(MapAuthorMetadata).ToDictionary(x => x.ForeignAuthorId, x => x);
+            var series = resource.Series.Select(MapSeries).ToList();
 
             foreach (var work in resource.Works)
             {