Merge pull request #1389 from abel533/default_name

注解去掉 default ""，必须指定任务名称。

xxl-job-core/src/main/java/com/xxl/job/core/handler/annotation/JobHandler.java

@@ -19,6 +19,6 @@ import java.lang.annotation.Target;
 @Deprecated
 public @interface JobHandler {
 
-    String value() default "";
+    String value();
 
 }

xxl-job-core/src/main/java/com/xxl/job/core/handler/annotation/XxlJob.java

@@ -15,7 +15,7 @@ public @interface XxlJob {
     /**
      * jobhandler name
      */
-    String value() default "";
+    String value();
 
     /**
      * init handler, invoked when JobThread init