Fixed: Inverted sections render on non-empty lists.

This fixes the following issue:

{{{
import mustache;
import std.stdio;

alias MustacheEngine!(string) Mustache;

void main()
{
    Mustache mustache;
    auto context = new Mustache.Context;
    context.addSubContext("section")["foo"] = "bar";
	
	auto mustacheTemplate = "{{^section}}This shouldn't be seen.{{/section}}";
    stdout.rawWrite(mustache.renderString(mustacheTemplate, context));
}
}}}

Expected output:
(empty)

Actual output:
{{{This shouldn't be seen.}}}