Normal Description
When the HML is <html><body/></html>, the content rewriter returns a null value, which is then displayed as "null" by the <hst:html> tag. Because it is hard to check before displaying it whether the HTML is empty or not, I think it should display an empty string instead of "null".
In HstHtmlTag class, something like that should do the job (not tested): writer.print(html != null ? html : "");