fix no span on execution context error

ddtrace/contrib/trace_utils.py

@@ -575,7 +575,10 @@ def activate_distributed_headers(tracer, int_config=None, request_headers=None,
         #     app = Flask(__name__)  # Traced via Flask instrumentation
         #     app = DDWSGIMiddleware(app)  # Extra layer on top for WSGI
         current_context = tracer.current_trace_context()
-        if current_context and current_context.trace_id and current_context.trace_id == context.trace_id:
+
+        # We accept incoming contexts with only baggage, however if we
+        # already have a current_context then a baggage only context will be tossed out
+        if current_context and (not context.trace_id or current_context.trace_id == context.trace_id):
             log.debug(
                 "will not activate extracted Context(trace_id=%r, span_id=%r), a context with that trace id is already active",  # noqa: E501
                 context.trace_id,