fix empty parentId in cards

- if the parentId is undefined or empty, the code run in a infinite loop and
  the browser "hangs"

models/cards.js

@@ -982,15 +982,16 @@ Cards.helpers({
   },
 
   parentCard() {
-    if (this.parentId === '') {
-      return null;
+    let ret = null;
+    if (this.parentId) {
+      ret = ReactiveCache.getCard(this.parentId);
     }
-    return ReactiveCache.getCard(this.parentId);
+    return ret;
   },
 
   parentCardName() {
     let result = '';
-    if (this.parentId !== '') {
+    if (this.parentId) {
       const card = ReactiveCache.getCard(this.parentId);
       if (card) {
         result = card.title;
@@ -1002,7 +1003,7 @@ Cards.helpers({
   parentListId() {
     const result = [];
     let crtParentId = this.parentId;
-    while (crtParentId !== '') {
+    while (crtParentId) {
       const crt = ReactiveCache.getCard(crtParentId);
       if (crt === null || crt === undefined) {
         // maybe it has been deleted
@@ -1022,7 +1023,7 @@ Cards.helpers({
     const resultId = [];
     const result = [];
     let crtParentId = this.parentId;
-    while (crtParentId !== '') {
+    while (crtParentId) {
       const crt = ReactiveCache.getCard(crtParentId);
       if (crt === null || crt === undefined) {
         // maybe it has been deleted
@@ -1048,7 +1049,11 @@ Cards.helpers({
   },
 
   isTopLevel() {
-    return this.parentId === '';
+    let ret = false;
+    if (this.parentId) {
+      ret = true;
+    }
+    return ret;
   },
 
   isLinkedCard() {