[5.7] Collection avg() should ignore null

[DianaDenpa]

The collection "avg()" method counts null as a value to divide by, resulting in an inaccurate calculation.

Examples:

• Before

$c = new Collection([1, 2, 3, 4, 5, null])
$c->avg(); // 2,5

• After

$c = new Collection([1, 2, 3, 4, 5, null])
$c->avg(); // 3

This PR is a follow up to #23909