drive: fix range requests on 0 length files

Before this change a range request on a 0 length file would fail

    $ rclone cat --head 128 drive:test/emptyfile
    ERROR : open file failed: googleapi: Error 416: Request range not satisfiable, requestedRangeNotSatisfiable

To fix this we remove Range: headers on requests for zero length files.

backend/drive/drive.go

@@ -2436,6 +2436,10 @@ func (o *baseObject) httpResponse(url, method string, options []fs.OpenOption) (
 		return req, nil, err
 	}
 	fs.OpenOptionAddHTTPHeaders(req.Header, options)
+	if o.bytes == 0 {
+		// Don't supply range requests for 0 length objects as they always fail
+		delete(req.Header, "Range")
+	}
 	err = o.fs.pacer.Call(func() (bool, error) {
 		res, err = o.fs.client.Do(req)
 		if err == nil {