PHP 8.4 Fixes: Implicitly marking parameter $block_visibility_group as nullable is deprecated

Problem/Motivation

A number of functions have implicit nullable values, which must be explicit for PHP 8.4.

Steps to reproduce

Set your php version to 8.4
Install the module
Run drush cr

Proposed resolution

Update function signatures in multiple classes to be explicitly ?[type] $[variable] = NULL
Additionally update code to take advantage of PHP strict typing and state-of-the-art PHP standards