DRV2SI

make context propagation robust to unavailability of root tracer

3

`opencensus-ext-threading` currently breaks when there is no tracer to propagate upon thread creation. This pull request makes it robust to this situation. There is also [an associated issue on GitHub](https://github.com/census-instrumentation/opencensus-python/issues/1191).

OpenCensus threading extension not graceful in case of missing global tracer

4

I ran into this situation in the context of Python Azure Functions, so I will stick to this context for explanation. ### TL;DR OpenCensus threading extension expects a global tracer...