Change ConversationId property to be nullable.

(cherry picked from commit 38aee33)

Author: yigolden

samples/KcpTunnel/TcpForwardConnection.cs

@@ -75,7 +75,7 @@ public async Task RunAsync()
             }
             finally
             {
-                _connection.UnregisterConversation(_conversation.ConversationId)?.Dispose();
+                _connection.UnregisterConversation(_conversation.ConversationId.GetValueOrDefault())?.Dispose();
                 _conversation.Dispose();
                 Console.WriteLine("Conversation closed: " + _conversation.ConversationId);
             }

samples/KcpTunnel/TcpSourceConnection.cs

@@ -71,9 +71,9 @@ public async Task RunAsync()
             }
             finally
             {
-                _connection.UnregisterConversation(_conversation.ConversationId)?.Dispose();
+                _connection.UnregisterConversation(_conversation.ConversationId.GetValueOrDefault())?.Dispose();
                 _conversation.Dispose();
-                _idPool.Return((ushort)_conversation.ConversationId);
+                _idPool.Return((ushort)_conversation.ConversationId.GetValueOrDefault());
                 Console.WriteLine("Conversation closed: " + _conversation.ConversationId);
             }
         }

src/KcpSharp/KcpConversation.cs

@@ -175,7 +175,7 @@ public void SetExceptionHandler(Func<Exception, KcpConversation, object?, bool>
         /// <summary>
         /// Get the ID of the current conversation.
         /// </summary>
-        public int ConversationId => (int)_id;
+        public int? ConversationId => (int)_id;
 
         /// <summary>
         /// Get whether the transport is marked as closed.

src/KcpSharp/KcpRawChannel.cs

@@ -82,7 +82,7 @@ public void SetExceptionHandler(Func<Exception, KcpRawChannel, object?, bool> ha
         /// <summary>
         /// Get the ID of the current conversation.
         /// </summary>
-        public int ConversationId => (int)_id;
+        public int? ConversationId => (int)_id;
 
         /// <summary>
         /// Get whether the transport is marked as closed.