Chunking no longer produces HTML for initial topic ref

[raducoravu]

Based on this report:
https://www.oxygenxml.com/forum/viewtopic.php?f=20&t=25830
chunk-introduction-issue.zip

If we have a DITA Map with topicrefs like this:

    <map>
        <title>Mathematics</title>
        <topicref href="introduction.dita"/>
        <topicref href="parent.dita" chunk="to-content" >
            <topicref href="introduction.dita"/>
        </topicref>
    </map>

The HTML output folder will not contain an "introduction.html" for the first topicref, instead the first link in the TOC will point to the "parent.html".