Feat: make the keyword compliant with OpenApi 3.1

[p-pichet]

When usign to validate a schema with a nullable array

class foo
{ 
    /** @var ?array $bar */
   protetected ?array $bar;
   }

The type is an array with the values array' and null` on the OpenApi 3.1

With this fix, we check if the type is string or an array.

[p-pichet]

@scaytrase I see you are the last that merge on this project.
Can we see if it's good ?