sdk: Span.resource should default to an empty resource

[toumorokoshi]

Right now, A resource object attached to a Span can be None:

https://github.com/open-telemetry/opentelemetry-python/blob/master/opentelemetry-sdk/src/opentelemetry/sdk/trace/__init__.py#L267

(also type is incorrect as it should also be a Resource object)

That means that there's two similar objects that refer to an empty resource: None, and the _EMPTY_RESOURCE global.

For simplicity I would argue that we should just always default to the _EMPTY_RESOURCE object: this avoids None checks in exporters.

We didn't use Resources in a meaningful way previously, which is why I believe this didn't come up before.