[2.5][PropertyAccess] Fixed getValue() when accessing non-existing indices of ArrayAccess implementations

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Previously, when the following code was executed:

```php
$object = new ImplOfArrayAccess();

$propertyAccessor->setValue($object, '[index]', 'Value');
```

and that index did not exist, a fatal error would be generated because `array_keys()` was executed on `$object`. This error is fixed now.