return array element if it exists, otherwise null
For the question title, there indeed a null coalescing operator introduced in PHP7:

return $array[$key] ?? null;


which is a shorthand for 

return (isset($array[$key])) ? $array[$key] : null;


however, in your code you are using empty() which is different from isset() and for this code there is no shorthand in PHP.

with empty(), your code will also return null if $array[$key] exists but contains an empty-like value, like false, 0, empty array and such