frontend: Ingress: Mark rules field as optional

[sniok]

This PR marks rules field in the Ingress object as optional, and updates usages accordingly

Related Issue

Fixes #3522

Steps to Test

1. Create Ingress without rules

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: my-ingress
spec:
  defaultBackend:
    service:
      name: my-service
      port:
        number: 80

2. Go to Map and make sure it doesn't crash

[skoeva]

thanks for this fix

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: skoeva, sniok

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• OWNERS [sniok]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment