Sampler.shouldSample can get a parent SpanContext of null

[Oberon00]

Samplers get null for their parent SpanContext in shouldSample if the parent is "invalid". This seems to be the only place in the whole SDK+API where a SpanContext can be null. Everywhere else we pass an invalid/empty context in such a case. I think the sampling API should behave the same way.

null assigned here if the parent is invalid:

opentelemetry-java/sdk/src/main/java/io/opentelemetry/sdk/trace/SpanBuilderSdk.java

Line 214 in 78ed649

parentContext = null;

then passed to shouldSample here:

opentelemetry-java/sdk/src/main/java/io/opentelemetry/sdk/trace/SpanBuilderSdk.java

Lines 231 to 232 in 78ed649

	.shouldSample(
	parentContext, traceId, spanName, spanKind, immutableAttributes, immutableLinks);

[jarebudev]

i can pick this one up