Fix requests integration when there is no tracer (#418)

Author: kornholi

opencensus/trace/ext/requests/trace.py

@@ -112,11 +112,13 @@ def wrap_session_request(wrapped, instance, args, kwargs):
     _span.name = '[requests]{}'.format(method)
     _span.span_kind = span_module.SpanKind.CLIENT
 
-    tracer_headers = _tracer.propagator.to_headers(
-        _tracer.span_context)
-
-    kwargs.setdefault('headers', {}).update(
-        tracer_headers)
+    try:
+        tracer_headers = _tracer.propagator.to_headers(
+            _tracer.span_context)
+        kwargs.setdefault('headers', {}).update(
+            tracer_headers)
+    except Exception:  # pragma: NO COVER
+        pass
 
     # Add the requests url to attributes
     _tracer.add_attribute_to_current_span(HTTP_URL, url)