Update the initial chunked attachment creation on the target to use an empty content

[Dedac]

When running a migration recently, the work items with attachments failed to upload the attachments with the error:

Unable to upload attachment EmploymentRefsScreen_inside-app_statuses.docx for source work item 35991 to the target account. The request indicated a Content-Type of "application/x-www-form-urlencoded" for method type "POST" which is not supported. Valid content types for this method are: application/json, application/json-patch+json, application/octet-stream.. The request indicated a Content-Type of "application/x-www-form-urlencoded" for method type "POST" which is not supported. Valid content types for this method are: application/json, application/json-patch+json, application/octet-stream.

I believe that there was a change to the HttpClient that sent "application/x-www-form-urlencoded" for empty content, which the Azure DevOps api was unhappy with.
This fixes that problem by adding an empty content to that first post for sending a chunked attachment.

[BraunFranklin]

I have been getting the same error and can confirm that the proposed changes did seem to resolve the issue.