Fix document elements check to prevent WYSIWYG error on IE

When creating a document using DomParser's parseFromString method from an empty string, IE 11
returns a document which both head and body elements are null, which caused the errors.

Fixes #13209

Author: Mateusz Krzeszowiak

lib/web/mage/adminhtml/wysiwyg/tiny_mce/plugins/magentovariable/editor_plugin.js

@@ -196,10 +196,8 @@ define([
                     }
                 });
 
-                returnval += doc.head.innerHTML ?
-                    doc.head.innerHTML.replace(/"/g, '"') : '';
-                returnval += doc.body.innerHTML ?
-                    doc.body.innerHTML.replace(/"/g, '"') : '';
+                returnval += doc.head ? doc.head.innerHTML.replace(/"/g, '"') : '';
+                returnval += doc.body ? doc.body.innerHTML.replace(/"/g, '"') : '';
 
                 return returnval ? returnval : content;
             },

lib/web/mage/adminhtml/wysiwyg/tiny_mce/plugins/magentowidget/editor_plugin.js

@@ -212,10 +212,8 @@ define([
                     widgetEl.parentNode.removeChild(widgetEl);
                 });
 
-                returnval += doc.head.innerHTML ?
-                    doc.head.innerHTML.replace(/"/g, '"') : '';
-                returnval += doc.body.innerHTML ?
-                    doc.body.innerHTML.replace(/"/g, '"') : '';
+                returnval += doc.head ? doc.head.innerHTML.replace(/"/g, '"') : '';
+                returnval += doc.body ? doc.body.innerHTML.replace(/"/g, '"') : '';
 
                 return returnval ? returnval : content;
             },