Fix webhook panic when VPC is nil in IBMPowerVSCluster spec

[Karthik-K-N]

What this PR does / why we need it:

This PR contains changes to avoid webhook panic

Before change

$ cat template.yaml
apiVersion: infrastructure.cluster.x-k8s.io/v1beta2
kind: IBMPowerVSCluster
metadata:
  annotations:
    powervs.cluster.x-k8s.io/create-infra: "true"
  labels:
    cluster.x-k8s.io/cluster-name: capi-powervs-karthik-two
  name: capi-powervs-karthik-two
  namespace: default
spec:
  serviceInstance:
    name: capi-powervs-karthik-two-serviceInstance
    
$ kubectl create -f template.yaml

Error from server (InternalError): error when creating "/Users/karthikkn/karthik-k8-workspace/cluster-api-provider-ibmcloud/templates/new.yaml": Internal error occurred: failed calling webhook "vibmpowervscluster.kb.io": failed to call webhook: Post "https://capi-ibmcloud-webhook-service.capi-ibmcloud-system.svc:443/validate-infrastructure-cluster-x-k8s-io-v1beta2-ibmpowervscluster?timeout=10s": EOF

After this fix

$ kubectl create -f template.yaml

The IBMPowerVSCluster "capi-powervs-karthik-two" is invalid:
* spec.zone: Invalid value: "null": value of zone is empty
* spec.vpc: Invalid value: "null": value of VPC is empty
* spec.resourceGroup: Invalid value: "null": value of resource group is empty

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

/area provider/ibmcloud

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Release note:

Fixed webhook panic when VPC is nil in IBMPowerVSCluster spec

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[Amulyam24]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Amulyam24, Karthik-K-N, mkumatag

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