Erroring out of _completeUploads if we get an empty response from S3.

lib/multipartupload.js

@@ -367,6 +367,7 @@ MultiPartUpload.prototype._completeUploads = function(callback) {
         // Register the response handler
         parse.xmlResponse(req, function(err, body) {
             if (err) return callback(err);
+            if (body === null) return callback('got empty response');
             delete body.$;
             body.size = size;
             mpu.emit('completed', body);