Make package compatible with PHP 8.4

[adrmrn]

Hi! I have upgraded to PHP 8.4 in my project recently, and I noticed a few deprecation notices, e.g.,

Deprecated: Qdrant\Qdrant::collections(): Implicitly marking parameter $collectionName as nullable is deprecated, the explicit nullable type must be used instead in .../vendor/hkulekci/qdrant/src/Qdrant.php on line 23

Deprecated: Qdrant\Http\Builder::__construct(): Implicitly marking parameter $client as nullable is deprecated, the explicit nullable type must be used instead in .../vendor/hkulekci/qdrant/src/Http/Builder.php on line 18

RFC: https://wiki.php.net/rfc/deprecate-implicitly-nullable-types

So I decided to fix the issues to get rid of notices. Changes are backward compatible.
I have also defined a job to run tests with PHP 8.4.

Let me know wdyt