ContinueFromHeaders should handle all combinations of sentry-trace/baggage properly

[tonyo]

At the moment ContinueFromHeaders function doesn't handle a case properly when sentry-trace is empty, and baggage is populated with some Sentry data.
In this case, the created span should have an empty unfrozen Dynamic Sampling Context, because the current SDK will essentially become the head SDK.

Extracted from #532:

	        // TODO(anton): we should handle this case properly
		// {
		// 	// No sentry-trace, but baggage with Sentry values => this is a head SDK, so the DSC
		// 	// should be empty and unfrozen.
		// 	traceStr:   "",
		// 	baggageStr: "sentry-trace_id=d49d9bf66f13450b81f65bc51cf49c03,sentry-public_key=public,sentry-sample_rate=1",
		// 	wantSpan: Span{
		// 		isTransaction: true,
		// 		dynamicSamplingContext: DynamicSamplingContext{
		// 			Frozen: false,
		// 		},
		// 	},
		// },