Only set trace and parent ID if it's passed

Otherwise we will potentially overwrite an auto-generated random ID

Author: andor44

ddtrace/contrib/flask/middleware.py

@@ -93,8 +93,10 @@ def _start_span(self, trace_id=None, parent_id=None):
                 service=self._service,
                 span_type=http.TYPE,
             )
-            g.flask_datadog_span.trace_id = trace_id
-            g.flask_datadog_span.parent_id = parent_id
+            if trace_id:
+                g.flask_datadog_span.trace_id = trace_id
+            if parent_id:
+                g.flask_datadog_span.parent_id = parent_id
         except Exception:
             self.app.logger.exception("error tracing request")