Number of replicas is needed

[rgordill]

Describe the bug
When deployed without it, the operator cannot create the controller.

To Reproduce
Steps to reproduce the behavior:

1. Deploy the following instance:

apiVersion: k8s.nginx.org/v1alpha1
kind: NginxIngressController
metadata:
  name: nginx-ingress-controller
  namespace: default
spec:
  type: deployment
  image:
    repository: nginx/nginx-ingress
    tag: 1.9.1
    pullPolicy: Always
  serviceType: NodePort
  enableCRDs: False
  nginxPlus: False

2. View logs on the operator.
3. See error like:

{"level":"error","ts":1614670022.4628859,"logger":"controller","msg":"Reconciler error","controller":"nginxingresscontroller-controller","name":"nginx-ingress-controller","namespace":"default","error":"NginxIngressController.k8s.nginx.org \"nginx-ingress-controller\" is invalid: spec.replicas: Invalid value: \"null\": spec.replicas in body must be of type integer: \"null\"","stacktrace":"github.com/go-logr/zapr.(*zapLogger).Error\n\tgithub.com/go-logr/zapr@v0.1.1/zapr.go:128\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).reconcileHandler\n\tsigs.k8s.io/controller-runtime@v0.6.3/pkg/internal/controller/controller.go:246\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).processNextWorkItem\n\tsigs.k8s.io/controller-runtime@v0.6.3/pkg/internal/controller/controller.go:218\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).worker\n\tsigs.k8s.io/controller-runtime@v0.6.3/pkg/internal/controller/controller.go:197\nk8s.io/apimachinery/pkg/util/wait.BackoffUntil.func1\n\tk8s.io/apimachinery@v0.18.9/pkg/util/wait/wait.go:155\nk8s.io/apimachinery/pkg/util/wait.BackoffUntil\n\tk8s.io/apimachinery@v0.18.9/pkg/util/wait/wait.go:156\nk8s.io/apimachinery/pkg/util/wait.JitterUntil\n\tk8s.io/apimachinery@v0.18.9/pkg/util/wait/wait.go:133\nk8s.io/apimachinery/pkg/util/wait.Until\n\tk8s.io/apimachinery@v0.18.9/pkg/util/wait/wait.go:90"}

Expected behavior
According with the docs, replicas shold be 1 if missing.

Your environment

• Version of the NGINX Ingress Operator - master
• Version of the Ingress Controller - 1.9.1
• Version of Kubernetes - 1.20.4
• Kubernetes platform - kubeadm
• Using NGINX