Refactor AsyncApiWalker to ensure null checks before walking references

Added explicit null checks for `reply.Address` and `reply.Channel`.
This prevents potential NullReferenceException when these properties are not initialized.
This change ensures that the walker safely handles cases where optional fields might be missing, improving robustness of the code.

Author: VisualBean

src/ByteBard.AsyncAPI/Services/AsyncApiWalker.cs

@@ -587,8 +587,15 @@ private void Walk(AsyncApiOperationReply reply)
 
             this.visitor.Visit(reply);
 
-            this.Walk(reply.Address);
-            this.Walk(reply.Channel as IAsyncApiReferenceable);
+            if (reply.Address != null)
+            {
+                this.Walk(reply.Address);
+            }
+
+            if (reply.Channel != null)
+            {
+                this.Walk(reply.Channel as IAsyncApiReferenceable);
+            }
 
             foreach (var message in reply.Messages)
             {
@@ -1211,4 +1218,4 @@ public void Walk(IAsyncApiElement element)
             }
         }
     }
-}
+}