Fix StoryboardResourceLookupStore dying on failure to unmap path

Before the introduction of `StoryboardResourceLookupStore`, missing
files would softly fail by use of null fallbacks. After the
aforementioned class was added, however, the fallbacks would not work
anymore if for whatever reason `GetStoragePathFromStoryboardPath()`
failed to unmap the storyboard asset name to a storage path.

osu.Game/Storyboards/Drawables/DrawableStoryboard.cs

@@ -142,14 +142,32 @@ public StoryboardResourceLookupStore(Storyboard storyboard, RealmAccess realm, G
             public void Dispose() =>
                 realmFileStore.Dispose();
 
-            public byte[] Get(string name) =>
-                realmFileStore.Get(storyboard.GetStoragePathFromStoryboardPath(name));
+            public byte[] Get(string name)
+            {
+                string? storagePath = storyboard.GetStoragePathFromStoryboardPath(name);
 
-            public Task<byte[]> GetAsync(string name, CancellationToken cancellationToken = new CancellationToken()) =>
-                realmFileStore.GetAsync(storyboard.GetStoragePathFromStoryboardPath(name), cancellationToken);
+                return string.IsNullOrEmpty(storagePath)
+                    ? null!
+                    : realmFileStore.Get(storagePath);
+            }
 
-            public Stream GetStream(string name) =>
-                realmFileStore.GetStream(storyboard.GetStoragePathFromStoryboardPath(name));
+            public Task<byte[]> GetAsync(string name, CancellationToken cancellationToken = new CancellationToken())
+            {
+                string? storagePath = storyboard.GetStoragePathFromStoryboardPath(name);
+
+                return string.IsNullOrEmpty(storagePath)
+                    ? Task.FromResult<byte[]>(null!)
+                    : realmFileStore.GetAsync(storagePath, cancellationToken);
+            }
+
+            public Stream? GetStream(string name)
+            {
+                string? storagePath = storyboard.GetStoragePathFromStoryboardPath(name);
+
+                return string.IsNullOrEmpty(storagePath)
+                    ? null
+                    : realmFileStore.GetStream(storagePath);
+            }
 
             public IEnumerable<string> GetAvailableResources() =>
                 realmFileStore.GetAvailableResources();