Check whether stack is non-empty before popping (exception and other listeners can stop event propagation)

Author: allflame

src/Bridge/RequestSpanListener.php

@@ -93,7 +93,6 @@ public function onRequest(GetResponseEvent $event)
                 ->addTag(new LongTag('time.micro', $startTime))
                 ->start($startTime);
         }
-
         $this->spans->push($requestSpan);
 
         return $this;