fix: nullref on walking optional reply property refs (#23)

* Fix: Try read relative uri's as files.

Signed-off-by: Alex Wichmann <VisualBean@users.noreply.github.com>

* Update src/ByteBard.AsyncAPI.Readers/Services/DefaultStreamLoader.cs

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>
Signed-off-by: Alex Wichmann <VisualBean@users.noreply.github.com>

* Apply suggestion from @VisualBean

Signed-off-by: Alex Wichmann <VisualBean@users.noreply.github.com>

* Apply suggestion from @VisualBean

Signed-off-by: Alex Wichmann <VisualBean@users.noreply.github.com>

* Apply suggestion from @VisualBean

Signed-off-by: Alex Wichmann <VisualBean@users.noreply.github.com>

* Apply suggestion from @VisualBean

Signed-off-by: Alex Wichmann <VisualBean@users.noreply.github.com>

* Refactor method signatures to include `baseUri` parameter

Updated all methods in `IStreamLoader`, `DefaultStreamLoader`, and related tests to accept both `baseUri` and `uri`.
Added a new property `BaseUri` to `AsyncApiReaderSettings` for resolving relative references.
Adjusted the implementation of external reference loading in `AsyncApiJsonDocumentReader`.

* 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.

---------

Signed-off-by: Alex Wichmann <VisualBean@users.noreply.github.com>
Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

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)
             }
         }
     }
-}
+}