Cluster CAPI operator webhook for Cluster object panics with empty spec

InfraStructureRef* is dereferenced without checking for nil value

Always

1. Run TechPreview cluster
2. Try to create Cluster object with empty spec
apiVersion: cluster.x-k8s.io/v1beta1
kind: Cluster
metadata:
  name: example
  namespace: openshift-cluster-api
spec: {}
 3. Observe panic in cluster-capi-operator

2023/03/10 14:13:31 http: panic serving 10.129.0.2:39614: runtime error: invalid memory address or nil pointer dereference
goroutine 3619 [running]:
net/http.(*conn).serve.func1()
    /usr/lib/golang/src/net/http/server.go:1850 +0xbf
panic({0x16cada0, 0x2948bc0})
    /usr/lib/golang/src/runtime/panic.go:890 +0x262
github.com/openshift/cluster-capi-operator/pkg/webhook.(*ClusterWebhook).ValidateCreate(0xc000ceac00?, {0x24?, 0xc00090fff0?}, {0x1b72d68?, 0xc0010831e0?})
    /go/src/github.com/openshift/cluster-capi-operator/pkg/webhook/cluster.go:32 +0x39
sigs.k8s.io/controller-runtime/pkg/webhook/admission.(*validatorForType).Handle(_, {_, _}, {{{0xc000ceac00, 0x24}, {{0xc00090fff0, 0x10}, {0xc000838000, 0x7}, {0xc000838007, ...}}, ...}})
    /go/src/github.com/openshift/cluster-capi-operator/vendor/sigs.k8s.io/controller-runtime/pkg/webhook/admission/validator_custom.go:79 +0x2dd
sigs.k8s.io/controller-runtime/pkg/webhook/admission.(*Webhook).Handle(_, {_, _}, {{{0xc000ceac00, 0x24}, {{0xc00090fff0, 0x10}, {0xc000838000, 0x7}, {0xc000838007, ...}}, ...}})
    /go/src/github.com/openshift/cluster-capi-operator/vendor/sigs.k8s.io/controller-runtime/pkg/webhook/admission/webhook.go:169 +0xfd
sigs.k8s.io/controller-runtime/pkg/webhook/admission.(*Webhook).ServeHTTP(0xc000630e80, {0x7f26f94b5580?, 0xc000f80280}, 0xc000750800)
    /go/src/github.com/openshift/cluster-capi-operator/vendor/sigs.k8s.io/controller-runtime/pkg/webhook/admission/http.go:98 +0xeb5
github.com/prometheus/client_golang/prometheus/promhttp.InstrumentHandlerInFlight.func1({0x7f26f94b5580, 0xc000f80280}, 0x1b7ff00?)
    /go/src/github.com/openshift/cluster-capi-operator/vendor/github.com/prometheus/client_golang/prometheus/promhttp/instrument_server.go:60 +0xd4
net/http.HandlerFunc.ServeHTTP(0x1b7ffb0?, {0x7f26f94b5580?, 0xc000f80280?}, 0x7afe60?)
    /usr/lib/golang/src/net/http/server.go:2109 +0x2f
github.com/prometheus/client_golang/prometheus/promhttp.InstrumentHandlerCounter.func1({0x1b7ffb0?, 0xc000a72000?}, 0xc000750800)
    /go/src/github.com/openshift/cluster-capi-operator/vendor/github.com/prometheus/client_golang/prometheus/promhttp/instrument_server.go:146 +0xb8
net/http.HandlerFunc.ServeHTTP(0x0?, {0x1b7ffb0?, 0xc000a72000?}, 0xc00056f0e1?)
    /usr/lib/golang/src/net/http/server.go:2109 +0x2f
github.com/prometheus/client_golang/prometheus/promhttp.InstrumentHandlerDuration.func2({0x1b7ffb0, 0xc000a72000}, 0xc000750800)
    /go/src/github.com/openshift/cluster-capi-operator/vendor/github.com/prometheus/client_golang/prometheus/promhttp/instrument_server.go:108 +0xbf
net/http.HandlerFunc.ServeHTTP(0xc000a72000?, {0x1b7ffb0?, 0xc000a72000?}, 0x18e45d1?)
    /usr/lib/golang/src/net/http/server.go:2109 +0x2f
net/http.(*ServeMux).ServeHTTP(0xc00056f0c0?, {0x1b7ffb0, 0xc000a72000}, 0xc000750800)
    /usr/lib/golang/src/net/http/server.go:2487 +0x149
net/http.serverHandler.ServeHTTP({0x1b71dc8?}, {0x1b7ffb0, 0xc000a72000}, 0xc000750800)
    /usr/lib/golang/src/net/http/server.go:2947 +0x30c
net/http.(*conn).serve(0xc00039af00, {0x1b81198, 0xc000416c00})
    /usr/lib/golang/src/net/http/server.go:1991 +0x607
created by net/http.(*Server).Serve
    /usr/lib/golang/src/net/http/server.go:3102 +0x4db

Webhook returns error, but does not panic