Error with empty anchors

When html content contains an empty anchor like the following

`some other thing`

an error is thrown:
`The value cannot be null.
Parameter name: elementId ( at TheArtOfDev.HtmlRenderer.Core.HtmlContainerInt.GetElementRectangle(String elementId)
at TheArtOfDev.HtmlRenderer.PdfSharp.HtmlContainer.GetElementRectangle(String elementId)
at TheArtOfDev.HtmlRenderer.PdfSharp.PdfGenerator.HandleLinks(PdfDocument document, HtmlContainer container, XSize orgPageSize, XSize pageSize)
at TheArtOfDev.HtmlRenderer.PdfSharp.PdfGenerator.AddPdfPages(PdfDocument document, String html, PdfGenerateConfig config, CssData cssData, EventHandler`1 stylesheetLoad, EventHandler`1 imageLoad)
at TheArtOfDev.HtmlRenderer.PdfSharp.PdfGenerator.GeneratePdf(String html, PageSize pageSize, Int32 margin, CssData cssData, EventHandler`1 stylesheetLoad, EventHandler`1 imageLoad)
`

Reproduce the failure with the HTML anchor shown in the issue and trace the PDF link-handling path beginning at PdfGenerator.HandleLinks and HtmlContainer.GetElementRectangle. Check how the empty fragment href is passed as elementId; done means PDF generation completes without the null-parameter exception for this input.

Scritto dal modello di indicizzazione a partire dal testo della issue.