Fixed cards export

Fixed cards export.

models/export.js

@@ -45,7 +45,7 @@ class Exporter {
 
   build() {
     const byBoard = { boardId: this._boardId };
-    const byBoardNoLinked = { boardId: this._boardId, linkedId: null };
+    const byBoardNoLinked = { boardId: this._boardId, linkedId: "" };
     // we do not want to retrieve boardId in related elements
     const noBoardId = { fields: { boardId: 0 } };
     const result = {