Description:
------------
As of PHP 7.4 using array_key_exists on objects is deprecated. The suggestion is to use
`isset` or `property_exists` instead.

The behaviour or both is different - one can detect `null` and the other can't.

Before in code we had:

```
$valueExists = array_key_exists($key, $data);
```

and it was working for any Traversable|array $data.

We are trying update our code and I thought it would be:

```
$valueExists = is_array($data) && array_key_exists($key, $data)
    || $data instanceof Traversable && property_exists($data, $key);
```

but unfortunately it is not working as expected.


Test script:
---------------
Please consider the following example:

```
$ao = new ArrayObject(['a' => null]);

var_dump($ao instanceof Traversable);  // true
var_dump(array_key_exists('a', $ao));  // true but reports deprecated notice on PHP 7.4+
var_dump(property_exists($ao, 'a'));   // false -> I would expect true here, as $ao is Traversable
```

Expected result:
----------------
`property_exists` works on Traversable as `array_key_exists` worked before.

Actual result:
--------------
`property_exists` does not work on every Traversable.

I've noted that it is possible to set `ArrayObject::ARRAY_AS_PROPS` flat on ArrayObject and this might be the solution. But honestly I am not sure if it is by design or should be fixed.