kong: only set `clusterIP` when `type` is `ClusterIP`

[pmalek]

What this PR does / why we need it:

Only set clusterIP when type is ClusterIP. This will allow using e.g. NodePort in ingress chart.

Which issue this PR fixes

• fixes #960

Special notes for your reviewer:

After this gets merged we can add a test case to ingress chart like so:

controller:
  ingressController:
    env:
      anonymous_reports: "false"
    enabled: true
    image:
      repository: kong/kubernetes-ingress-controller

    gatewayDiscovery:
      enabled: true
    adminApi:
      tls:
        client:
          enabled: true

gateway:
  env:
    anonymous_reports: "off"
  admin:
    type: NodePort
  readinessProbe:
    initialDelaySeconds: 1
    timeoutSeconds: 1
    periodSeconds: 1

Checklist

• [x] PR is based off the current tip of the main branch.
• [x] Changes are documented under the "Unreleased" header in CHANGELOG.md
• [x] New or modified sections of values.yaml are documented in the README.md
• [x] Commits follow the Kong commit message guidelines

[pmalek]

Ah yes, null does work 🤦 Approved #962