set_correlation_ids should handle the empty context case

[joeyzhao2018]

What does this PR do?

• When neither x-ray nor dd-trace is used, the tracing context should be {} and set_correlation_ids() should not break. That is ,

if not context:
    return

• Reorganized the code for get_dd_trace_context but no logic change.

Motivation

#332

Testing Guidelines

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
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[duncanista]

LGTM!