Fixed: Custom script execution when release processed via /push API

Co-Authored-By: Mark McDowall <markus101@users.noreply.github.com>

src/NzbDrone.Core/Notifications/CustomScript/CustomScript.cs

@@ -44,7 +44,7 @@ namespace NzbDrone.Core.Notifications.CustomScript
             environmentVariables.Add("Lidarr_Release_AlbumReleaseDates", string.Join(",", remoteAlbum.Albums.Select(e => e.ReleaseDate)));
             environmentVariables.Add("Lidarr_Release_AlbumTitles", string.Join("|", remoteAlbum.Albums.Select(e => e.Title)));
             environmentVariables.Add("Lidarr_Release_Title", remoteAlbum.Release.Title);
-            environmentVariables.Add("Lidarr_Release_Indexer", remoteAlbum.Release.Indexer);
+            environmentVariables.Add("Lidarr_Release_Indexer", remoteAlbum.Release.Indexer ?? string.Empty);
             environmentVariables.Add("Lidarr_Release_Size", remoteAlbum.Release.Size.ToString());
             environmentVariables.Add("Lidarr_Release_Quality", remoteAlbum.ParsedAlbumInfo.Quality.Quality.Name);
             environmentVariables.Add("Lidarr_Release_QualityVersion", remoteAlbum.ParsedAlbumInfo.Quality.Revision.Version.ToString());