Tracer::getGlobalTracer() must return null tracer

[beberlei]

As per go implementation, the default global tracer is noop, so our PHP Tracer::getGlobalTracer() must be:

abstract class Tracer
{
    public function getGlobalTracer()
    {
        if (self::$globalTracer === null) {
            self::$globalTracer = new Stub\Tracer();
        }
        return self::$globalTracer;
    }
}

[wladekb]

Done.