Improve set-constant.js scriptlet

The scriptlet will now still try to trap a specific
property if a segment of the chain is not undefined
while yet not an object either.

For example, this now allows to set a value on
`document.body.onselectstart` when `document.body` has
not been instantiated yet by the browser parser,
whereas this would previously fail because
`document.body` would be `null` while the scriptlet
was testing against `undefined`.

assets/resources/scriptlets.js

@@ -549,7 +549,7 @@
         const prop = chain.slice(0, pos);
         let v = owner[prop];
         chain = chain.slice(pos + 1);
-        if ( v !== undefined ) {
+        if ( v instanceof Object ) {
             makeProxy(v, chain);
             return;
         }