handle nil *HTML in render (#138)
handle nil *HTML in render (#138)

This change fixes a regression introduced by PR #134.

The basic issue is that when `nil` is encountered, it is treated correctly as a nil render,
but when `(*HTML)(nil)` is encountered, it is not. I discovered this while porting
Go-Package-Store (which uses Vecty) over to the new API.

The following code effectively demonstrates the issue:

```Go
package main

import (
	"github.com/gopherjs/vecty"
	"github.com/gopherjs/vecty/elem"
)

func main() {
	vecty.RenderBody(&PageView{})
}

type PageView struct {
	vecty.Core
}

func (p *PageView) Render() *vecty.HTML {
	return elem.Body(
		// this works:
		//nil,

		// this does not:
		p.returnsNil(),
	)
}

func (p *PageView) returnsNil() *vecty.HTML {
	return nil
}
```

gopherjs/vecty