HttpTraceContext injects invalid span context

[Flarna]

Please answer these questions before submitting a bug report.

What version of OpenTelemetry are you using?

0.18.2

What version of Node are you using?

14.16.0

Please provide the code you used to setup the OpenTelemetry SDK

const tp = new NodeTracerProvider();
tp.addSpanProcessor(new SimpleSpanProcessor(new ZipkinExporter({ serviceName: "Foo" })));
tp.register();

What did you do?

Create an invalid/NoopSpan and inject it via propagator.

const noopSpan = NOOP_TRACER.startSpan("NoOp");
const carrier = {};
propagation.inject(setSpan(ROOT_CONTEXT, noopSpan), carrier);
console.log(carrier);

What did you expect to see?

Invalid SpanContext get's not injected into carrier.

What did you see instead?

Invalid span context gets injected:

{
  traceparent: '00-00000000000000000000000000000000-0000000000000000-00'
}

Additional context

I know that noop span creation via NOOP_TRACER is not a real world usecase. But NOOP_TRACER is used at several places in SDK therefore this can happen. But my target here was to create a simple reproducer.