Fix various failures in initial statistics fetch

- If the local user is restricted, then attempting to fetch their data
  from the `/users` endpoint would result in an empty response.

- Even if the user was successfully fetched, their `RulesetsStatistics`
  may not be populated (and instead be `null`). Curiously this was not
  picked up by static analysis until the first issue was fixed.

Closes #21839.

osu.Game/Online/Solo/SoloStatisticsWatcher.cs

@@ -75,15 +75,27 @@ namespace osu.Game.Online.Solo
                 return;
 
             var userRequest = new GetUsersRequest(new[] { localUser.OnlineID });
-            userRequest.Success += response => Schedule(() =>
+            userRequest.Success += initialiseUserStatistics;
-            {
-                latestStatistics = new Dictionary<string, UserStatistics>();
-                foreach (var rulesetStats in response.Users.Single().RulesetsStatistics)
-                    latestStatistics.Add(rulesetStats.Key, rulesetStats.Value);
-            });
             api.Queue(userRequest);
         });
 
+        private void initialiseUserStatistics(GetUsersResponse response) => Schedule(() =>
+        {
+            var user = response.Users.SingleOrDefault();
+
+            // possible if the user is restricted or similar.
+            if (user == null)
+                return;
+
+            latestStatistics = new Dictionary<string, UserStatistics>();
+
+            if (user.RulesetsStatistics != null)
+            {
+                foreach (var rulesetStats in user.RulesetsStatistics)
+                    latestStatistics.Add(rulesetStats.Key, rulesetStats.Value);
+            }
+        });
+
         private void userScoreProcessed(int userId, long scoreId)
         {
             if (userId != api.LocalUser.Value?.OnlineID)