Fixed: Calendar in Work Effort does not work (OFBIZ-12665)

Fix a null pointer exception when a menu extend a menu with empty item.
In this case, we use a empty link for the extended menu.

Thanks to Jacques Leroux for the alert
diff --git a/framework/widget/src/main/java/org/apache/ofbiz/widget/model/CommonWidgetModels.java b/framework/widget/src/main/java/org/apache/ofbiz/widget/model/CommonWidgetModels.java
index f92c2d2..b05c631 100644
--- a/framework/widget/src/main/java/org/apache/ofbiz/widget/model/CommonWidgetModels.java
+++ b/framework/widget/src/main/java/org/apache/ofbiz/widget/model/CommonWidgetModels.java
@@ -444,6 +444,32 @@
             this.height = "";
         }
 
+        // Empty link constructor
+        public Link() {
+            this.autoEntityParameters = null;
+            this.autoServiceParameters = null;
+            this.callback = null;
+            this.encode = false;
+            this.fullPath = false;
+            this.idExdr = FlexibleStringExpander.getInstance("");
+            this.image = null;
+            this.linkType = "";
+            this.nameExdr = FlexibleStringExpander.getInstance("");
+            this.parameterList = Collections.emptyList();
+            this.prefixExdr = FlexibleStringExpander.getInstance("");
+            this.secure = false;
+            this.styleExdr = FlexibleStringExpander.getInstance("");
+            this.targetExdr = FlexibleStringExpander.getInstance("");
+            this.targetWindowExdr = FlexibleStringExpander.getInstance("");
+            this.textExdr = FlexibleStringExpander.getInstance("");
+            this.urlMode = "intra-app";
+            this.size = null;
+            this.requestConfirmation = false;
+            this.confirmationMsgExdr = FlexibleStringExpander.getInstance("");
+            this.width = "";
+            this.height = "";
+        }
+
         public AutoEntityParameters getAutoEntityParameters() {
             return autoEntityParameters;
         }
diff --git a/framework/widget/src/main/java/org/apache/ofbiz/widget/model/ModelMenuItem.java b/framework/widget/src/main/java/org/apache/ofbiz/widget/model/ModelMenuItem.java
index 3ebb034..e9b1455 100644
--- a/framework/widget/src/main/java/org/apache/ofbiz/widget/model/ModelMenuItem.java
+++ b/framework/widget/src/main/java/org/apache/ofbiz/widget/model/ModelMenuItem.java
@@ -693,7 +693,7 @@
             this.linkMenuItem = parentMenuItem;
             this.link = originLink != null
                     ? originLink.link
-                    : null;
+                    : new Link();
         }
 
         public MenuLink(GenericValue portalPage, ModelMenuItem parentMenuItem, Locale locale) {