PHP 8.4 compatibility

[andypost]

RFC https://php.watch/rfcs/deprecate-implicitly-nullable-types

Testing via docker

expander$ docker run --rm -v $(pwd):/mnt -w /mnt php:8.4.0alpha2-cli-alpine find . -type f -name '*.php' -exec php -l {} \;
No syntax errors detected in ./tests/src/ExpanderTest.php
No syntax errors detected in ./src/StringifierInterface.php

Deprecated: Grasmash\Expander\Expander::doExpandArrayProperties(): Implicitly marking parameter $reference_data as nullable is deprecated, the explicit nullable type must be used instead in ./src/Expander.php on line 102

Deprecated: Grasmash\Expander\Expander::expandStringPropertiesCallback(): Implicitly marking parameter $reference_data as nullable is deprecated, the explicit nullable type must be used instead in ./src/Expander.php on line 201
No syntax errors detected in ./src/Expander.php
No syntax errors detected in ./src/Stringifier.php