ENH PHP 8.1 compatibility

[emteknetnz]

Issue silverstripe/silverstripe-framework#10250

[GuySartorelli]

This is a really long PR so I stopped marking them pretty quickly but there are a lot of examples of the below regex functions getting an empty string where an empty regex is needed. Probably worth amending the automated thing and re-running it rather than trying to go through them all by hand.

[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, but I'll hold off merging until the outstanding questions about file_get_contents(), and file_put_contents(), and unlink() have been answered on the other PRs.
Edit: Question was answered. There is no problem - the approach here is correct.