Stop handling the case when foo.Spec.Replicas is nil

[shuheiktgw]

Sorry for nitpicking but I believe we don't need to handle the case when foo.Spec.Replicas is nil since foo.Spec.Replicas is required is a required field and cannot be nil 😄

[govargo]

Thank you for this PR.

I added this case because the code of some branches removed this required marker.

e.g.
https://github.com/govargo/foo-controller-kubebuilder/blob/admission_webhook/api/v1alpha1/foo_types.go#L33-L36
https://github.com/govargo/foo-controller-kubebuilder/blob/multi_conversion/api/v1alpha1/foo_types.go#L33-L36

This is why it was necessary for some cases to match the story flow of the book.

[shuheiktgw]

@govargo So sorry for not noticing your comment 🙇
That makes sense so let me close this PR!