Fix season handling ("Season Unknown" / unneccesary empty seasons) (#12240)

MediaBrowser.Controller/Entities/TV/Series.cs

@@ -350,10 +350,17 @@ namespace MediaBrowser.Controller.Entities.TV
 
         public List<BaseItem> GetSeasonEpisodes(Season parentSeason, User user, DtoOptions options, bool shouldIncludeMissingEpisodes)
         {
+            var queryFromSeries = ConfigurationManager.Configuration.DisplaySpecialsWithinSeasons;
+
+            // add optimization when this setting is not enabled
+            var seriesKey = queryFromSeries ?
+                GetUniqueSeriesKey(this) :
+                GetUniqueSeriesKey(parentSeason);
+
             var query = new InternalItemsQuery(user)
             {
-                AncestorWithPresentationUniqueKey = null,
-                SeriesPresentationUniqueKey = GetUniqueSeriesKey(this),
+                AncestorWithPresentationUniqueKey = queryFromSeries ? null : seriesKey,
+                SeriesPresentationUniqueKey = queryFromSeries ? seriesKey : null,
                 IncludeItemTypes = new[] { BaseItemKind.Episode },
                 OrderBy = new[] { (ItemSortBy.SortName, SortOrder.Ascending) },
                 DtoOptions = options