Fix JS object checking when visiting a key path
Fix JS object checking when visiting a key path

When reactiveObj contains a null value, attempting to get children of the null
value results in an error.

So now we check using `x instanceof Object` instead of `typeof x === 'object'`
so that NOTSET will be returned when a parent in the key path is null.

xamfoo/reactive-obj,xamfoo/reactive-obj