Fix admission-controller panic

When applying a regular VPA, this panis.

```
2026/04/10 15:33:35 http: panic serving 172.18.0.2:33326: runtime error: invalid memory address or nil pointer dereference
goroutine 196 [running]:
net/http.(*conn).serve.func1()
	/usr/local/go/src/net/http/server.go:1907 +0xac
panic({0xe0ffc0?, 0x1b88340?})
	/usr/local/go/src/runtime/panic.go:860 +0x12c
k8s.io/autoscaler/vertical-pod-autoscaler/pkg/admission-controller/resource/vpa.validatePerVPAFeatureFlag(0x5288459fab00)
	/workspace/pkg/admission-controller/resource/vpa/handler.go:268 +0x58
k8s.io/autoscaler/vertical-pod-autoscaler/pkg/admission-controller/resource/vpa.ValidateVPA(0x5288459fab00, 0x1)
	/workspace/pkg/admission-controller/resource/vpa/handler.go:112 +0x2c
k8s.io/autoscaler/vertical-pod-autoscaler/pkg/admission-controller/resource/vpa.(*resourceHandler).GetPatches(0x528845bdabb0, {0x528845e1e7e0?, 0x528845fc9880?}, 0xfee800?)
	/workspace/pkg/admission-controller/resource/vpa/handler.go:82 +0x9c
k8s.io/autoscaler/vertical-pod-autoscaler/pkg/admission-controller/logic.(*AdmissionServer).admit(0x52884591aa08, {0x116b770, 0x528845cd4550}, {0x528845e79b00, 0x8e5, 0x900})
	/workspace/pkg/admission-controller/logic/server.go:125 +0x114
k8s.io/autoscaler/vertical-pod-autoscaler/pkg/admission-controller/logic.(*AdmissionServer).Serve(0x52884591aa08, {0x1169a00, 0x528845f841e0}, 0x528845b1c8c0)
	/workspace/pkg/admission-controller/logic/server.go:196 +0x254
main.main.func1({0x1169a00?, 0x528845f841e0?}, 0x528845fbbb18?)
	/workspace/pkg/admission-controller/main.go:119 +0x3c
net/http.HandlerFunc.ServeHTTP(0x1baffa0?, {0x1169a00?, 0x528845f841e0?}, 0x528845fbbb00?)
	/usr/local/go/src/net/http/server.go:2286 +0x38
net/http.(*ServeMux).ServeHTTP(0x10?, {0x1169a00, 0x528845f841e0}, 0x528845b1c8c0)
	/usr/local/go/src/net/http/server.go:2828 +0x190
net/http.serverHandler.ServeHTTP({0x11632b8?}, {0x1169a00?, 0x528845f841e0?}, 0x1?)
	/usr/local/go/src/net/http/server.go:3311 +0xb0
net/http.(*conn).serve(0x528845c22ab0, {0x116b738, 0x528845e1ec60})
	/usr/local/go/src/net/http/server.go:2073 +0x51c
created by net/http.(*Server).Serve in goroutine 1
	/usr/local/go/src/net/http/server.go:3464 +0x37c
```

Author: adrianmoisey

vertical-pod-autoscaler/pkg/admission-controller/resource/vpa/handler.go

@@ -265,7 +265,7 @@ func validateMemoryResolution(val apires.Quantity) error {
 
 func validatePerVPAFeatureFlag(vpa *vpa_types.VerticalPodAutoscaler) error {
 	featureFlagOn := features.Enabled(features.PerVPAConfig)
-	if !featureFlagOn && vpa.Spec.UpdatePolicy.EvictAfterOOMSeconds != nil {
+	if !featureFlagOn && vpa.Spec.UpdatePolicy != nil && vpa.Spec.UpdatePolicy.EvictAfterOOMSeconds != nil {
 		return fmt.Errorf("EvictAfterOOMSeconds is not supported when feature flag %s is disabled", features.PerVPAConfig)
 	}