[]struct{} leads to incorrectly generated code

[echlebek]

The attached example leads to code that does not build. (Go 1.9 on my machine, but almost certainly on all release versions of Go.)

package ffjsonbug

type Foo struct {
       	Bar []struct{}
}

Here is the relevant section of generated code:

       	if j.Bar != nil {
       		buf.WriteString(`[`)
       		for i, v := range j.Bar {
       			if i != 0 {
       				buf.WriteString(`,`)
       			}
       			/* Inline struct. type=struct {} kind=struct */
       			buf.WriteString(`{}`)
       		}
       		buf.WriteString(`]`)
       	} else {
       		buf.WriteString(`null`)
       	}

The special case for struct{} causes v to be un-used.