Fix an DOMDocument error with NoseTestEngine running on PHP 7

Summary:
In php 7, DOMDocument::loadXML emits an error when supplied with
an empty string as input. For example, I got this error:

ERROR 2: DOMDocument::loadXML(): Empty string supplied as input

This change simply checks for empty and returns an empty array
rather than attempting to parse an empty xml document.

Test Plan: ran arc diff on a repo that uses nosetestengine

Reviewers: #blessed_reviewers!

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D16672