Fix rabbitmq NullPointerException (#12109)

Author: huange7

instrumentation/rabbitmq-2.7/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/rabbitmq/RabbitChannelInstrumentation.java

@@ -128,6 +128,9 @@ public static void stopSpan(
       if (callDepth.decrementAndGet() > 0) {
         return;
       }
+      if (scope == null) {
+        return;
+      }
 
       scope.close();