Problem/Motivation
In recent config validation issues we've been changing some string values to support NULLs where the empty string does not make sense. For example, #3437325: Add validation constraints to system.date made system.date:country nullable and in #3443432: Add validation constraints to system.site a lot of the string values in system.site are being made nullable.
The problem with making these values nullable is that PHP 8 has got progressively stricter about what can be passed into string functions as NULL will often produce a deprecation and PHP 9 will error.
Proposed resolution
Decide on a policy.