Array diff problem with one array contains null, and other empty values

I am using laravel. I have 2 arrays. For example

$array1 = [
    'test1',
    '',
    'test2',
    'test3',
    '',
    'test4',
    '',
];

$array2 = [
    null
];

dd(array_diff($array1, $array2));

result is

array:4 [▼
  0 => "test1"
  2 => "test2"
  3 => "test3"
  5 => "test4"
]

but I expected

array:7 [▼
  0 => "test1"
  1 => ""
  2 => "test2"
  3 => "test3"
  4 => ""
  5 => "test4"
  6 => ""
]

Can you ask why in array_diff considered null is equal '', and how can I fix it without any additional cods

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