Replace instanceof Object with typeof checks

Using `instanceof Object` is generally problematic, since it's not guaranteed to always do the right thing for all Objects.
(I stumbled upon this while working on another patch, when I noticed that the `outlineView` was broken with workers disabled.)

src/core/xfa/xfa_object.js

@@ -586,7 +586,7 @@ class XFAObject {
     if (Array.isArray(obj)) {
       return obj.map(x => XFAObject[_cloneAttribute](x));
     }
-    if (obj instanceof Object) {
+    if (typeof obj === "object" && obj !== null) {
       return Object.assign({}, obj);
     }
     return obj;