Fix nullable parameter

[VincentLanglet]

Hi @matthewelwell

With PHP 8.4 we get deprecation like

Flagsmith\Flagsmith::__construct(): Implicitly marking parameters $apiKey, $host, $customHeaders, $environmentTtl, $retries, $defaultFlagHandler, and $offlineHandler as nullable is deprecated, the explicit nullable type must be used instead.

I try to solve all of them.

[matthewelwell]

@VincentLanglet thanks for submitting this. The PR looks good to me. You'll need to rebase onto the main branch once #91 is merged and then we can get this merged in.

[matthewelwell]

@VincentLanglet #91 was merged yesterday so you're good to rebase and fix the conflict, then we can get this merged. Thanks!

[VincentLanglet]

Done @matthewelwell

[VincentLanglet]

Hi @matthewelwell I'm not sure why but the 4.5.1 release is not present on composer
https://packagist.org/packages/flagsmith/flagsmith-php-client

[matthewelwell]

@VincentLanglet one day I'll remember to bump the version in composer.json (or we'll eventually integrate release please into this repo!).

Sorry, it should be there now (or very soon) - I've recreated the release after bumping the version in #92