fix(core): Adds ability to store null value in ordered hash set (#447)

- [X] Adds the ability to store a null value in an ordered hash set

TODO:
Optimized the OrderedHashSet. See this:
* https://github.com/dotnet/runtime/issues/10050
Looks like the OrderedDictionary that I based this structure from was not updated.
See the source for diffing:
https://github.com/dotnet/runtime/blob/master/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/HashSet.cs

Difference between a HashSet and an OrderedHashSet is updating the indexes of the entries after fixing the chain to preserve insertion order.

Projects/Server.Tests/Tests/Collections/OrderedHashSetTests.cs

@@ -9,7 +9,10 @@ namespace Server.Tests
         [Fact]
         public void TestOrderedHashSet()
         {
-            var set = new OrderedHashSet<string>(1) { "random string1", "another random string1", "another random string2", "another random string3" };
+            var set = new OrderedHashSet<string>(1)
+            {
+                "random string1", "another random string1", "another random string2", "another random string3"
+            };
             set.Remove("another random string1");
             var list = set.ToList();
 
@@ -20,5 +23,24 @@ namespace Server.Tests
                 Assert.Equal(entry, arr[i++]);
             }
         }
+
+        [Fact]
+        public void TestOrderedHashWithNull()
+        {
+            var set = new OrderedHashSet<string>(1)
+            {
+                "random string1", null, "another random string2", "another random string3"
+            };
+
+            set.Remove("another random string1");
+            var list = set.ToList();
+
+            string[] arr = { "random string1", null, "another random string2", "another random string3" };
+            int i = 0;
+            foreach (var entry in list)
+            {
+                Assert.Equal(entry, arr[i++]);
+            }
+        }
     }
 }