Remove implicitly nullable parameter declarations

[MarkusBauer]

Implicitly nullable parameter declarations are deprecated from PHP 8.4 on and will be removed in PHP 9. This project uses them in one function only, which is fixed by this PR.

How to reproduce:

> docker run --rm -v .:/app "php:8.4-rc-cli" find /app -type f -name "*.php" -exec php -d error_reporting=32765 -l {} \; | grep -v 'No syntax errors detected'
Deprecated: Jenssegers\ImageHash\ImageHash::__construct(): Implicitly marking parameter $implementation as nullable is deprecated, the explicit nullable type must be used instead in /app/src/ImageHash.php on line 16
Deprecated: Jenssegers\ImageHash\ImageHash::__construct(): Implicitly marking parameter $driver as nullable is deprecated, the explicit nullable type must be used instead in /app/src/ImageHash.php on line 16

Note: Not fixed are deprecated features in the test dependencies (namely PhpCoveralls).

[pataar]

@jenssegers Any chance in merging this?