make context propagation robust to unavailability of root tracer

[DRV2SI]

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.

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[DRV2SI]

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[eliMsbitSoftware]

this issue is a blocker in our project, any chance this could be reviewed and considered? @DRV2SI have you found new workarounds since?