Replace last remaining __proto__: empty hack

We want to allow passing objects with null or normal object prototypes,
and were using `__proto__: empty` to indicate that, but I'm not
convinced this is at all logical, and does cause issues in some cases
(see Flow issue 8685).

Author: mwiencek

root/static/scripts/common/utility/hasOwnProp.js

@@ -11,7 +11,7 @@
 const objectHasOwnProperty = Object.prototype.hasOwnProperty;
 
 export default function hasOwnProp(
-  object: {__proto__: empty, ...},
+  object: {__proto__: null, ...} | {...},
   prop: string,
 ): boolean {
   return objectHasOwnProperty.call(object, prop);