ENH PHP 8.1 compatibility

[emteknetnz]

Issue silverstripe/silverstripe-framework#10250

[GuySartorelli]

For posterity: See discussion in silverstripe/silverstripe-admin#1294 about the general approach. tl;dr:

the purposes of this PR is to avoid passing null (specifically null) from built-in functions that as of php 8.1 will throw deprecation warnings if null is passed in. The approach is necessarily heavy-handed, and while there are many situations where it isn't needed, it would be prohibitively laborious to find all of those situations by hand.

Looks good to me, though I don't have access to merge this one.

[tractorcow]

I've tagged 1.0.0 fyi

[emteknetnz]

@tractorcow any chance you could tag 0.1.3? - proxy-db requires ^0.1.1 https://github.com/tractorcow/silverstripe-proxy-db/blob/master/composer.json

[emteknetnz]

@tractorcow I've opened up a subsequent pull-request on proxy-db to require ^1 of classproxy