Return context with no Baggage if unable to extract

[MrAlias]

From the specification:

If the propagator is unable to parse the incoming baggage, extract MUST return a Context with no baggage entries in it.

Currently we do not clear the baggage from the parent context passed as an argument.

opentelemetry-go/propagation/baggage.go

Line 105 in bb4c297

return parent

Similarly,

If the incoming baggage is present, but contains no entries, extract MUST return a Context with no baggage entries in it.

and we do not guarantee this:

opentelemetry-go/propagation/baggage.go

Lines 60 to 62 in bb4c297

	if bVal == "" {
	return parent
	}

This would seem to be out of compliance with the specification. However, it does not seem like ideal behavior to clear valid existing baggage. This PR will determine if we need to clear existing baggage.

[MrAlias]

The requirement that a context needs to be cleared of existing Baggage if it fails to extract/inject was a bug in the specification, it is specified more generally in the API Propagators specification that that behavior must not take place. Our implementation is compliant with the resolution specified in open-telemetry/opentelemetry-specification#1575