Fix daily challenge not showing a replay button in results screen

[frenzibyte]

The results screen pushed by DailyChallenge always load with a null selected score, so the replay download button never gets added. There's no reason not to have the replay download button visible when there's no score selected, so just...add it?

Failing test coverage was reverted due to out-of-scope issues (see discord).

Before:

After:

[bdach]

Generally after looking this over from fifty sides (that's what happens with eldritch abstract entity hierarchies with everything overloaded to crap) I think this may be fine, and the nullcheck was just overeager application of NRT (blames back to da953b3).

However I am slightly unnerved by some stuff ReplayDownloadButton is doing in the Score value change callback and would probably want to see something like the following happen:

diff --git a/osu.Game/Screens/Ranking/ReplayDownloadButton.cs b/osu.Game/Screens/Ranking/ReplayDownloadButton.cs
index aac29ad269..5e2161c251 100644
--- a/osu.Game/Screens/Ranking/ReplayDownloadButton.cs
+++ b/osu.Game/Screens/Ranking/ReplayDownloadButton.cs
@@ -88,6 +88,8 @@ private void load(OsuGame? game, ScoreModelDownloader scoreDownloader)
                 State.ValueChanged -= exportWhenReady;
 
                 downloadTracker?.RemoveAndDisposeImmediately();
+                downloadTracker = null;
+                State.SetDefault();
 
                 if (score.NewValue != null)
                 {

just to make sure that if score changes from non-null to null, then the State of the button reverts to the default rather than staying at whatever the last score had, because if that is allowed to happen, then stuff like the export flow / exportWhenReady() suddenly start looking rather unsafe.

That could be worth a test case. Something like this is enough to show what I mean:

diff --git a/osu.Game.Tests/Visual/Gameplay/TestSceneReplayDownloadButton.cs b/osu.Game.Tests/Visual/Gameplay/TestSceneReplayDownloadButton.cs
index 5b32f380b9..061e8ea7e1 100644
--- a/osu.Game.Tests/Visual/Gameplay/TestSceneReplayDownloadButton.cs
+++ b/osu.Game.Tests/Visual/Gameplay/TestSceneReplayDownloadButton.cs
@@ -117,6 +117,9 @@ public void TestButtonWithReplayStartsDownload()
 
             AddAssert("state entered downloading", () => downloadStarted);
             AddUntilStep("state left downloading", () => downloadFinished);
+
+            AddStep("change score to null", () => downloadButton.Score.Value = null);
+            AddUntilStep("state changed to unknown", () => downloadButton.State.Value, () => Is.EqualTo(DownloadState.Unknown));
         }
 
         [Test]

In theory if you have a download button in LocallyAvailable state, then reset the score to null and then request an export, it'll die.