array_column() null index key bug

1-example.php

<?php

$a = [ 
    ['a' => 10],
    ['a' => 20],
    ['a' => null],
    ['a' => null],
];

$b = [ 
    ['a' => 'foo'],
    ['a' => 'bar'],
    ['a' => null],
    ['a' => null],
];

$c = [
    ['a' => 10],
    ['a' => 20],
    ['a' => null],
    ['a' => null],
    ['a' => 'foo'],
    ['a' => 'bar'],
    ['a' => null],
    ['a' => null],
];

var_export(array_column($a, null, 'a'));
echo "\n\n";
var_export(array_column($b, null, 'a'));
echo "\n\n";
var_export(array_column($c, null, 'a'));

2-actual-output.txt

array (
  10 =>
  array (
    'a' => 10,
  ),
  20 =>
  array (
    'a' => 20,
  ),
  21 =>
  array (
    'a' => NULL,
  ),
  22 =>
  array (
    'a' => NULL,
  ),
)

array (
  'foo' =>
  array (
    'a' => 'foo',
  ),
  'bar' =>
  array (
    'a' => 'bar',
  ),
  0 =>
  array (
    'a' => NULL,
  ),
  1 =>
  array (
    'a' => NULL,
  ),
)

array (
  10 =>
  array (
    'a' => 10,
  ),
  20 =>
  array (
    'a' => 20,
  ),
  21 =>
  array (
    'a' => NULL,
  ),
  22 =>
  array (
    'a' => NULL,
  ),
  'foo' =>
  array (
    'a' => 'foo',
  ),
  'bar' =>
  array (
    'a' => 'bar',
  ),
  23 =>
  array (
    'a' => NULL,
  ),
  24 =>
  array (
    'a' => NULL,
  ),
)

3-expected-output.txt

array (
  10 =>
  array (
    'a' => 10,
  ),
  20 =>
  array (
    'a' => 20,
  ),
  '' =>
  array (
    'a' => NULL,
  ),
)

array (
  'foo' =>
  array (
    'a' => 'foo',
  ),
  'bar' =>
  array (
    'a' => 'bar',
  ),
  '' =>
  array (
    'a' => NULL,
  ),
)

array (
  10 =>
  array (
    'a' => 10,
  ),
  20 =>
  array (
    'a' => 20,
  ),
  '' =>
  array (
    'a' => NULL,
  ),
  'foo' =>
  array (
    'a' => 'foo',
  ),
  'bar' =>
  array (
    'a' => 'bar',
  ),
)