Description:
------------
This problem occurs in Version 7.3.0 - 7.40beta4.

Steps to reproduce:
1. Have an array $a with elements and integer keys
2. Let the internal pointer point beyond the end of the elements list using next($a)
3. Set a value by key where key is lower then the lowest existing key in $a
4. Call key($a)

I expect key($a) to return the key of the element I just added (like it did before 7.3.0), but it returns null.

I have prepared a 3v4l script that shows the problem:
https://3v4l.org/DoFve

Thanks for having a look at this. I can provide more context if needed.

Test script:
---------------
$data = [1 => true];
next($data);
var_dump(key($data));
$data[0] = true;
var_dump(key($data));

Expected result:
----------------
NULL
int(0)

Actual result:
--------------
NULL
NULL