Fix last commit re. set-constant scriptlet

Related commit:
- 40ea9d69d5d0

Related issue:
- uBlockOrigin/uBlock-issues#947

Restore intended behavior with original commit:
- 14ebfbea279c

The purpose of the original change was to be able to
trap properties which values were `null`.

assets/resources/scriptlets.js

@@ -549,7 +549,10 @@
         const prop = chain.slice(0, pos);
         let v = owner[prop];
         chain = chain.slice(pos + 1);
-        if ( v instanceof Object || typeof v === 'object' ) {
+        if (
+            (v instanceof Object) ||
+            (typeof v === 'object' && v !== null)
+        ) {
             makeProxy(v, chain);
             return;
         }