Fixing PHP 8.4 Implicitly nullable parameter declarations deprecated

[mafftor]

This dependency In PHP 8.4 now throws deprecation:

Fluent\Logger\FluentLogger::__construct(): Implicitly marking parameter $packer as nullable is deprecated, the explicit nullable type must be used instead

Recommendation from PHP 8.4: Implicitly nullable parameter declarations deprecated the solution is to explicitly mark the parameter as nullable, or avoid implicit nullability.

To maintain compatibility with PHP 5.6 while addressing this deprecation, I opted to:

• Remove the type hint for the $packer parameter to avoid the nullable deprecation in PHP 8.4.
• Manually check the $packer parameter type within the constructor, ensuring it either implements PackerInterface or is null. If it's not, an InvalidArgumentException is thrown.

This change ensures the code works as expected across all supported PHP versions, including PHP 5.6 and PHP 8.4.

[brianlmoon]

It would be nice to have this pulled in ASAP.