Extract certain keys from array

Question

I have this array:

$array1 = [
    "a" => 1,
    "b" => 2,
    "c" => 3
];

Sometimes, in this array, the keys a and b are set to null, so the array is like the following:

$array2 = [
    "a" => null,
    "b" => null,
    "c" => null
];

How can i get a new array with only keys a and b if they're not null, otherwise an empty array?

This should be the result:

$newArray1 = [
    "a" => 1,
    "b" => 2
];

$newArray2 = [];