Fix possible nullref

Emby.Server.Implementations/TV/TVSeriesManager.cs

@@ -206,12 +206,14 @@ namespace Emby.Server.Implementations.TV
                     DtoOptions = dtoOptions
                 }).Cast<Episode>().FirstOrDefault();
 
+                if (!(nextEpisode is null)) {
                     var userData = _userDataManager.GetUserData(user, nextEpisode);
 
                     if (userData.PlaybackPositionTicks > 0)
                     {
                         return null;
                     }
+                }
 
                 return nextEpisode;
             };