change NULL_VALUE

mall-search/src/main/java/com/uas/search/util/ObjectToDocumentUtils.java

@@ -21,7 +21,7 @@ public class ObjectToDocumentUtils {
     /**
      * 空值
      */
-    public static final String NULL_VALUE = "NULL_VALUE";
+    public static final String NULL_VALUE = "NULLVALUE";
 
 	/**
 	 * 将对象转为Document