Fix NPE from SimpleTracer#createBaggage

[ttddyy]

I was writing a unittest with SimpleTracer.

Following this baggage API documentation:

@Test
void foo() {
  Tracer tracer = new SimpleTracer();
  Span span = tracer.nextSpan().name("parent").start();

// Assuming that there's a span in scope...
  try (Tracer.SpanInScope ws = tracer.withSpan(span)) {

    // Not passing a TraceContext explicitly will bind the baggage to the
    // current TraceContext
    Baggage baggageForSpanInScopeOne = tracer.createBaggage("from_span_in_scope 1", "value 1");
    ...
  }
}

This gets NPE while creating a baggage.

It is because in SimpleBaggageManager

tracing/micrometer-tracing-tests/micrometer-tracing-test/src/main/java/io/micrometer/tracing/test/simple/SimpleBaggageManager.java

Lines 66 to 69 in c93a098

	private SimpleBaggageInScope baggageForName(TraceContext traceContext, String name) {
	return this.baggagesByContext.get(traceContext).stream().filter(bag -> name.equalsIgnoreCase(bag.name()))
	.findFirst().orElse(null);
	}

The this.baggagesByContext.get(traceContext) returns null. Then trying to stream() causes NPE.

This PR changes to handle the case when baggagesByContext map does not have a corresponding traceContext.
The callers of this method already handle null return value.

[shakuzen]

Looks good to me. Thank you. Could you rebase and target the 1.0.x branch?