Nil pointer dereference, on an invalid spec

[mjnovice]

/kind bug

What steps did you take and what happened:
[A clear and concise description of what the bug is.]
I created a GCPManagedCluster without the network field and the controller crashed.

What did you expect to happen:
The controller shouldn't crash

https://github.com/kubernetes-sigs/cluster-api-provider-gcp/blob/main/cloud/services/container/clusters/reconcile.go#L256

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• Cluster-api version:
• Minikube/KIND version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):