rest: fix upload of 0 length files

Before this change if ContentLength was set in the options but 0 then
we would upload using chunked encoding.  Fix this to always upload
with a "Content-Length" header even if the size is 0.

Remove workarounds for this from b2 and onedrive backends.

This fixes the issue for the webdav backend described here:

https://forum.rclone.org/t/code-500-errors-with-webdav-nextcloud/8440/

backend/onedrive/onedrive.go

@@ -1560,10 +1560,6 @@ func (o *Object) uploadSinglepart(in io.Reader, size int64, modTime time.Time) (
 		}
 	}
 
-	if size == 0 {
-		opts.Body = nil
-	}
-
 	err = o.fs.pacer.Call(func() (bool, error) {
 		resp, err = o.fs.srv.CallJSON(&opts, nil, &info)
 		if apiErr, ok := err.(*api.Error); ok {