old() uses array_key_exists instead of isset().

Old() uses `isset()` to check if a property exists. However, the problem is that sometimes we legitimately have the value of `null` for a property and the property does exist.

If `isset` is used, if the property exists but the value is `null`, nothing is returned. I updated it to use `array_key_exists()` as this tells us whether the property exists or not and does not care about the value.