Fix participant panel null user test no longer functioning properly

I guess the changes that involved `MultiplayerTestScene` having a test
user lookup cache caused this test case to false-pass silently.

Added an explicit assert which ensures the added user indeed has a null
`User` value.

osu.Game/Tests/Visual/TestUserLookupCache.cs

@@ -10,10 +10,22 @@ namespace osu.Game.Tests.Visual
 {
     public class TestUserLookupCache : UserLookupCache
     {
-        protected override Task<User> ComputeValueAsync(int lookup, CancellationToken token = default) => Task.FromResult(new User
+        /// <summary>
+        /// A special user ID which <see cref="ComputeValueAsync"/> would return a <see langword="null"/> <see cref="User"/> for.
+        /// As a simulation to what a regular <see cref="UserLookupCache"/> would return in the case of failing to fetch the user.
+        /// </summary>
+        public const int NULL_USER_ID = -1;
+
+        protected override Task<User> ComputeValueAsync(int lookup, CancellationToken token = default)
         {
-            Id = lookup,
-            Username = $"User {lookup}"
-        });
+            if (lookup == NULL_USER_ID)
+                return Task.FromResult((User)null);
+
+            return Task.FromResult(new User
+            {
+                Id = lookup,
+                Username = $"User {lookup}"
+            });
+        }
     }
 }