Fix error when season stats are missing

src/NzbDrone.Api/Series/SeriesModule.cs

@@ -167,11 +167,14 @@ namespace NzbDrone.Api.Series
             resource.PreviousAiring = seriesStatistics.PreviousAiring;
             resource.SizeOnDisk = seriesStatistics.SizeOnDisk;
 
+            if (seriesStatistics.SeasonStatistics != null)
+            {
                foreach (var season in resource.Seasons)
                 {
                     season.Statistics = seriesStatistics.SeasonStatistics.SingleOrDefault(s => s.SeasonNumber == season.SeasonNumber).InjectTo<SeasonStatisticsResource>();
                 }
             }
+        }
 
         private void PopulateAlternateTitles(List<SeriesResource> resources)
         {