I got an 'System.InvalidOperationException' and message 'Cannot save a PDF document with no pages.'

I generate a PdfDocument with ```TheArtOfDev.HtmlRenderer.PdfSharp.PdfGenerator.GeneratePdf(html, PdfSharp.PageSize.Letter)```, and the html has contents, but the PdfDocument is empty and cause a 'System.InvalidOperationException' when i save the document by ```pdf.Save(memoryStream)```

Start at TheArtOfDev.HtmlRenderer.PdfSharp.PdfGenerator.GeneratePdf and follow the returned PdfDocument through pdf.Save(memoryStream). Reproduce the empty-document failure with the HTML input described in the issue, then identify why no pages are created. Done means a document generated from HTML with contents can be saved without the InvalidOperationException.

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