admission/webhook: fix panic from empty response in mutating webhooks

staging/src/k8s.io/apiserver/pkg/admission/plugin/webhook/mutating/dispatcher.go

@@ -20,6 +20,7 @@ package mutating
 
 import (
 	"context"
+	"fmt"
 	"time"
 
 	jsonpatch "github.com/evanphx/json-patch"
@@ -88,6 +89,10 @@ func (a *mutatingDispatcher) callAttrMutatingHook(ctx context.Context, h *v1beta
 		return &webhookerrors.ErrCallingWebhook{WebhookName: h.Name, Reason: err}
 	}
 
+	if response.Response == nil {
+		return &webhookerrors.ErrCallingWebhook{WebhookName: h.Name, Reason: fmt.Errorf("Webhook response was absent")}
+	}
+
 	if !response.Response.Allowed {
 		return webhookerrors.ToStatusErr(h.Name, response.Response.Result)
 	}