[10.x] Allow default values when merging values into a resource

[axlon]

This PR adds the option to enter a default when merging data into a resource. If the condition is not met and no default value is given a MissingValue instance will be returned, just like before.

Before:

class MyResource extends JsonResource
{
    public function toArray($request): array
    {
        return [
            /* other properties */
            $this->mergeWhen($this->hasSpecificData(), $this->specificData()),
            $this->mergeUnless($this->hasSpecificData(), static::genericData()),
        ];
    }
}

After:

class MyResource extends JsonResource
{
    public function toArray($request): array
    {
        return [
            /* other properties */
            $this->mergeWhen($this->hasSpecificData(), $this->specificData(), static::genericData()),
        ];
    }
}