Add null to array_map(null, $a, $b)

[schlndh]

Fixes: https://3v4l.org/QKHrt (https://phpstan.org/r/357b09e6-80b7-4c50-97db-35995ff4cb67)

Unfortunately, since it depends on values of multiple variables at the same time (which may be correlated), there will be false positives like this:

if (rand()) {
	$a = [1];
	$b = [2];
} else {
	$a = [1, 2];
	$b = [3, 4];
}

array_map(null, $a, $b);

But I guess it's better to be safe than sorry. I at least added support for array_map(null, $a, $a).