Fix RabbitMq NPE case (#2450)

* Fix npe

apm-sniffer/apm-sdk-plugin/rabbitmq-5.x-plugin/src/main/java/org/apache/skywalking/apm/plugin/rabbitmq/RabbitMQConsumerInterceptor.java

@@ -51,7 +51,11 @@ public class RabbitMQConsumerInterceptor implements InstanceMethodsAroundInterce
         CarrierItem next = contextCarrier.items();
         while (next.hasNext()) {
             next = next.next();
-            next.setHeadValue(properties.getHeaders().get(next.getHeadKey()).toString());
+            if (properties.getHeaders() != null && properties.getHeaders().get(next.getHeadKey()) != null) {
+                next.setHeadValue(properties.getHeaders().get(next.getHeadKey()).toString());
+            } else {
+                next.setHeadValue("");
+            }
         }
         ContextManager.extract(contextCarrier);