If an empty xml-block is rendered via ez_render_field, the raw markup is:
<div class="ezxmltext-field"><?xml version="1.0" encoding="utf-8"?><section xmlns:image="http://ez.no/namespaces/ezpublish3/image/" xmlns:xhtml="http://ez.no/namespaces/ezpublish3/xhtml/" xmlns:custom="http://ez.no/namespaces/ezpublish3/custom/"/></div>
If the xml-block is not empty, the xml-node is not in the raw output.