Don't set tags which are None

This is often `None`, but tags are always strings, and so things get
broken when spans get passed along to other client calls.

Fixes open-telemetry#262

exporter/opentelemetry-exporter-datadog/src/opentelemetry/exporter/datadog/propagator.py

@@ -103,7 +103,7 @@ def inject(
             self.SAMPLING_PRIORITY_KEY,
             str(constants.AUTO_KEEP if sampled else constants.AUTO_REJECT),
         )
-        if constants.DD_ORIGIN in span.context.trace_state:
+        if span.context.trace_state.get(constants.DD_ORIGIN) is not None:
             setter.set(
                 carrier,
                 self.ORIGIN_KEY,