fix: Fixes PooledRefList ToList returning wrong size (#1899)

### Summary

- Fixed a bug caused by a bad assumption. If `m_List[index]` is sparse and null values are casted, the server does not crash.
- Fixed a bug where `PooledRefList.ToList` extension method returned the wrong list size.

Projects/UOContent/Gumps/AdminGump.cs

@@ -1274,7 +1274,10 @@ namespace Server.Gumps
                              i < 9 && index >= 0 && index < m_List.Count;
                              ++i, ++index)
                         {
-                            var a = (Account)m_List[index];
+                            if (m_List[index] is not Account a)
+                            {
+                                continue;
+                            }
 
                             var offset = 200 + i * 20;