fix: declare strong type for null apikey and apisecret

[shakaran]

Force strong type via constructor and attributes declaration. Avoid inconsitences with null values allowed in contructor

[p-delorme]

Thanks for the PR @shakaran. With this said, I might actually restructure this class to BaseServiceClass to make it easier and have less code duplication.

[shakaran]

@p-delorme as you want it. We have other possible PR to suggest to improve unit test and code. Also using phpstan, rector, php-cs-fixer and others in default pipeline

[p-delorme]

I've updated all these types in the constructors.

I'll close this, feel free to reopen if you wish.