fix(deployment): guard nil Spec.Replicas before dereference

[immanuwell]

What this PR does / why we need it:

Spec.Replicas is *int32 - a pointer. Three metrics dereference it without a nil check:

• kube_deployment_spec_replicas
• kube_deployment_spec_strategy_rollingupdate_max_unavailable
• kube_deployment_spec_strategy_rollingupdate_max_surge

replicaset.go and statefulset.go both guard this same field already. Deployment was just missing the guards, so it would straight up panic on a nil pointer if a deployment with unset Spec.Replicas came through (e.g. via direct API/etcd write, old migrated objects, or tests).

Reproduce:

d := &v1.Deployment{
    ObjectMeta: metav1.ObjectMeta{Name: "test", Namespace: "default"},
    Spec: v1.DeploymentSpec{
        Strategy: v1.DeploymentStrategy{
            RollingUpdate: &v1.RollingUpdateDeployment{
                MaxUnavailable: &maxUnavailable,
            },
        },
        // Replicas intentionally nil
    },
}
// kube-state-metrics panics when generating metrics for this object

Fix follows the existing pattern from replicaset.go:160 and statefulset.go:170.

How does this change affect the cardinality of KSM: does not change cardinality

Which issue(s) this PR fixes:

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