fix: when node is root, not output unnecessary string

[leavesster]

When I use OutputXML to copy a xml document I found When the node is root , this function will print unnecessary <> and <>。

s := `<?xml version="1.0" encoding="utf-8"?><rss version="2.0"></rss>`
doc, err := xmlquery.Parse(strings.NewReader(s))

log.Printf(doc.OutputXML(true))
// will print this:
//`<><?xml version="1.0" encoding="utf-8"?><rss version="2.0"></rss></>`

I not sure, if this is run as you expected. I think this may be not right behavior, so I create this pr.

[zhengchun]

Merged. thanks!