XFA - Fix error when creating a new data node

- fix for issue #13556;
  - value in a field can be empty.

src/core/xfa/som.js

@@ -313,7 +313,7 @@ function createDataNode(root, container, expr) {
       }
       root = child;
     } else {
-      parsed[i].index = children.length - index;
+      parsed[i].index = index - children.length;
       return createNodes(root, parsed.slice(i));
     }
   }

src/core/xfa/template.js

@@ -2396,9 +2396,15 @@ class Field extends XFAObject {
       if (this.ui.imageEdit) {
         ui.children.push(this.value[$toHTML]().html);
       } else if (!this.ui.button) {
-        const value = this.value.exData
-          ? this.value.exData[$text]()
-          : this.value[$toHTML]().html.value;
+        let value = "";
+        if (this.value.exData) {
+          value = this.value.exData[$text]();
+        } else {
+          const htmlValue = this.value[$toHTML]().html;
+          if (htmlValue !== null) {
+            value = htmlValue.value;
+          }
+        }
         if (value) {
           if (ui.children[0].name === "textarea") {
             ui.children[0].attributes.textContent = value;

test/pdfs/xfa_issue13556.pdf.link

@@ -0,0 +1 @@
+https://web.archive.org/web/20210422084343/https://mail.gnome.org/archives/evince-list/2013-November/pdfNC0hkVW67a.pdf

test/test_manifest.json

@@ -938,6 +938,14 @@
        "enableXfa": true,
        "type": "eq"
     },
+    {  "id": "xfa_issue13556",
+       "file": "pdfs/xfa_issue13556.pdf",
+       "md5": "197e93a010763c3b6f9845595ee66c70",
+       "link": true,
+       "rounds": 1,
+       "enableXfa": true,
+       "type": "eq"
+    },
     {  "id": "xfa_issue13213",
        "file": "pdfs/xfa_issue13213.pdf",
        "md5": "8a0e3179bffbac721589d1b1df863b49",