Fix GH-23576: array_keys() on an empty array returns a non-zero next index

[lazerg]

array_keys() returns the input array itself when that array is empty. An array that has held elements keeps its next free index, so array_keys() on an empty-but-previously-populated array hands back a next index that is not 0, and the first append lands on the old index instead:

$a = [123 => 123];
unset($a[123]);
$b = array_keys($a);
$b[] = 42;      // ends up at key 124, not 0

The shortcut has been there since 7.2, but it only became visible in 7.3.17 / 7.4.5, when the fix for bug #79364 made copying an empty array preserve the next free index. array_keys() is documented to return a list, so returning the caller's array is wrong regardless of what the copy preserves. Return a fresh empty array instead, which is what array_values() right below already does for the same case.

Fixes GH-23576

[devnexen]

would be nice to also test an empty array ?