Fix placeholder drawables on beatmap listing not always hiding correctly

`BeatmapListingOverlay.addContentToPlaceholder()`, in order to make
transitions between different beatmap listing content (whether it is
actual cards, or placeholders for no beatmaps found/supporter-specific
filters chosen), would set `BypassAutoSizeAxes = Y` on content as it is
fading out, to make the transition smoother. The property in question
was supposed to be getting restored to `None` on the next show.

In testing scenarios, it sometimes turned out that this wasn't the case,
therefore making the placeholders effectively not show - while they
were present and fully opaque, they would be the only child of
an auto-sized container with `BypassAutoSizeAxes = Y`, so the parent
auto-sized to a zero height, which logically follows from the premise,
but is not what was desired.

This in turn was caused by the fact that the `BypassAutoSizeAxes = Y`
set was scheduled, and sometimes it would be scheduled in such a way
that the drawable would cease to be present on the next frame due to its
alpha being past the cutoff point of 0.0001. Therefore the scheduled set
would not execute until the *next* time the placeholder was shown,
therefore causing the bug.

Fix by ensuring that the placeholder drawables are always present if
their schedulers have any tasks enqueued, on top of the usual checks of
alpha and scale performed via the base implementation.

osu.Game/Overlays/BeatmapListingOverlay.cs

@@ -217,6 +217,10 @@ namespace osu.Game.Overlays
 
         public class NotFoundDrawable : CompositeDrawable
         {
+            // required for scheduled tasks to complete correctly
+            // (see `addContentToPlaceholder()` and the scheduled `BypassAutoSizeAxes` set during fade-out in outer class above)
+            public override bool IsPresent => base.IsPresent || Scheduler.HasPendingTasks;
+
             public NotFoundDrawable()
             {
                 RelativeSizeAxes = Axes.X;
@@ -261,6 +265,10 @@ namespace osu.Game.Overlays
         // (https://github.com/ppy/osu-framework/issues/4530)
         public class SupporterRequiredDrawable : CompositeDrawable
         {
+            // required for scheduled tasks to complete correctly
+            // (see `addContentToPlaceholder()` and the scheduled `BypassAutoSizeAxes` set during fade-out in outer class above)
+            public override bool IsPresent => base.IsPresent || Scheduler.HasPendingTasks;
+
             private LinkFlowContainer supporterRequiredText;
 
             public SupporterRequiredDrawable()