chore: updates from code review

Co-authored-by: Shiwei Zhang <[email protected]>
Signed-off-by: Leonardo Chaia <[email protected]>

src/OrasProject.Oras/Registry/Remote/Repository.cs

@@ -347,6 +347,6 @@ internal Reference ParseReferenceFromContentReference(string reference)
     /// <param name="getContent"></param>
     /// <param name="cancellationToken"></param>
     /// <returns></returns>
-    public async Task MountAsync(Descriptor descriptor, string fromRepository, Func<CancellationToken, Task<Stream>>? getContent, CancellationToken cancellationToken = default) 
+    public async Task MountAsync(Descriptor descriptor, string fromRepository, Func<CancellationToken, Task<Stream>>? getContent = null, CancellationToken cancellationToken = default) 
         => await ((IMounter)Blobs).MountAsync(descriptor, fromRepository, getContent, cancellationToken).ConfigureAwait(false);
 }