Prevent error when attempting to use None context. (#541)

Author: purple4reina

datadog_lambda/tracing.py

@@ -699,6 +699,8 @@ def inject_correlation_ids():
 
 
 def set_dd_trace_py_root(trace_context_source, merge_xray_traces):
+    if not _is_context_complete(dd_trace_context):
+        return
     if trace_context_source == TraceContextSource.EVENT or merge_xray_traces:
         context = Context(
             trace_id=dd_trace_context.trace_id,

tests/test_tracing.py

@@ -934,6 +934,9 @@ def setUp(self):
         self.mock_activate = patcher.start()
         self.mock_activate.return_value = True
         self.addCleanup(patcher.stop)
+        patcher = patch("datadog_lambda.tracing.dd_trace_context", None)
+        self.mock_dd_trace_context = patcher.start()
+        self.addCleanup(patcher.stop)
 
     def tearDown(self):
         global dd_tracing_enabled
@@ -991,6 +994,10 @@ def test_set_dd_trace_py_root_no_span_id(self):
         self.mock_activate.assert_called()
         self.mock_activate.assert_has_calls([call(expected_context)])
 
+    def test_set_dd_trace_py_root_none_context(self):
+        set_dd_trace_py_root(TraceContextSource.EVENT, True)
+        self.mock_activate.assert_not_called()
+
 
 class TestServiceMapping(unittest.TestCase):
     def setUp(self):