handle null index param

Author: Cory Isakson

clients/dotnet/WebClient/MemoryWebClient.cs

@@ -242,7 +242,8 @@ public async Task<StreamableFileContent> ExportFileAsync(
         string? index = null,
         CancellationToken cancellationToken = default)
     {
-        string requestUri = $"{Constants.HttpDownloadEndpoint}?index={index}&documentId={documentId}&filename={fileName}";
+        string indexParam = index != null ? $"index={index}&" : "";
+        string requestUri = $"{Constants.HttpDownloadEndpoint}?{indexParam}documentId ={documentId}&filename={fileName}";
         HttpResponseMessage? response = await this._client.GetAsync(requestUri, cancellationToken).ConfigureAwait(false);
 
         response.EnsureSuccessStatusCode();