Use xray context span id when it exists

[purple4reina]

What does this PR do?

Address conditional runtime error when the xray context doesn't exist.

See original PR #400

Motivation

Encountered this error when running the extension on a lambda with SAM local.

[ERROR] 2023-11-29T01:44:35.868Z        56c0ddf8-9de9-4006-89a0-e46b7cace0f8    Error 'NoneType' object has no attribute 'span_id'. Traceback: Traceback (most recent call last):
  File "/var/task/datadog_lambda/wrapper.py", line 288, in _before
    set_dd_trace_py_root(trace_context_source, self.merge_xray_traces)
  File "/var/task/datadog_lambda/tracing.py", line 607, in set_dd_trace_py_root
    if xray_context.span_id:
AttributeError: 'NoneType' object has no attribute 'span_id'

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
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)

[duncanista]

LGTM!