PDFUAauto leaves a document without a title instead of taking one from <title> or the first heading

[jakejackson1]

On #325 (feature/pdfua-1-support), a document without a title under PDFUAauto only records a warning, and the output fails veraPDF ua1 7.1-9 (the metadata must contain dc:title).

$mpdf = new \Mpdf\Mpdf(['PDFUA' => true, 'PDFUAauto' => true]);
$mpdf->WriteHTML('<h1>Annual report</h1><p>Body</p>');

Expected. Auto mode fixes what it can. It could fall back to the HTML <title> if one was written, then to the text of the first heading, and warn only when neither exists. #324 does something similar for PDF/X, titling the document after its output file.