Support zero length uploads

[creadth]

Currently if UploadLength is set to zero without setting IsUploadDeferLength to true, the library will throw an error that "UploadLength should not be less than zero", so it is not possible to upload empty files without adjusting headers with OnPreSendRequestAsync event.

[bluetianx]

Why do you want to set UploadLength to zero without setting IsUploadDeferLength? Can you tell me about it?

[creadth]

When user uploads a file with zero content, BirdMessenger is unable to process this request and fails. tus server and other libs handle this request correctly - uploading an empty file.

[bluetianx]

ok, I got it , I will deal with it

[bluetianx]

[tus-resumable-upload-protocol](https://github.com/tus/tus-resumable-upload-protocol](https://github.com/tus/tus-resumable-upload-protocol/issues/196)

[bluetianx]

@creadth , Upgrading BirdMessenger to 3.1.3 , Your problem will be resolved