feat: allow setting array default value to null

[vkarpov15]

Fix #6691

Summary

I took another look at #6691 and I can't see any reason for us continuing to disallow default: null on array paths. We already allow default: undefined and default: () => null, the latter of which is practically identical. Silently ignoring default: null for arrays feels more like a bug than anything, and changing it doesn't break any tests. 8.5 release is a good opportunity to make this change.

Examples

[hasezoey]

LGTM, it does not break any tests, but it would likely be good to have new tests for this