Fix crash for ePub 3s with an empty nav table

We came across an ePub3 which crashed Readium and traced it back to the handling of an empty nav table. This pull request is for the fix I found to work. The ePub passes an epubcheck and the following is the contents of the nav html (I cannot share the entire ePub as it is copyrighted material):

    <?xml version="1.0" encoding="utf-8"?>
    <!DOCTYPE html>
    <html xml:lang="en-us" lang="en-us" xmlns="http://www.w3.org/1999/xhtml" xmlns:epub="http://www.idpf.org/2007/ops">
        <head>
            <title>Empty TOC</title>
            <meta charset="utf-8" />
        </head>
        <body>
            <nav epub:type="toc" id="toc">
                <ol>
                </ol>
            </nav>
        </body>
    </html>