Implement defensive programming techniques for setting spans in ContextVars

[duck-nukem]

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to avoid accidentally trying to merge a None with a tuple of Spans

What does this pull request do?

Simply adds a safe fallback in case the existing tuple of spans is None

Unfortunately I wasn't able to find the root cause of it, so I only treated the symptoms

Related issues

Closes #2056

[cla-checker-service[bot]]

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