fixed provider set delay NullPointerException (#3957)

* fixed provider set delay NullPointerException

* add getActualDelay method

* simple

Author: Leishunyu

dubbo-config/dubbo-config-api/src/main/java/org/apache/dubbo/config/ServiceConfig.java

@@ -360,6 +360,7 @@ private boolean shouldDelay() {
         if (delay == null && provider != null) {
             delay = provider.getDelay();
         }
+        this.delay = delay;
         return delay != null && delay > 0;
     }