fix machine image maintenance for shoots with multiple worker pools

```improvement operator
Fixes a bug in the maintenance controller that could lead to machine images to not be updated if the Shoot has multiple worker pools.
```

pkg/controllermanager/controller/shoot/shoot_maintenance_control.go

@@ -318,8 +318,10 @@ func hasMaintainNowAnnotation(shoot *gardencorev1beta1.Shoot) bool {
 
 // MaintainMachineImages determines if a shoots machine images have to be maintained and in case returns the target images
 func MaintainMachineImages(logger *logrus.Entry, shoot *gardencorev1beta1.Shoot, cloudProfile *gardencorev1beta1.CloudProfile) (updatedMachineImages []*gardencorev1beta1.ShootMachineImage, reasons []string, error error) {
-	var shootMachineImagesForUpdate []*gardencorev1beta1.ShootMachineImage
-	var reasonsForUpdate []string
+	var (
+		shootMachineImagesForUpdate []*gardencorev1beta1.ShootMachineImage
+		reasonsForUpdate            []string
+	)
 	for _, worker := range shoot.Spec.Provider.Workers {
 		workerImage := worker.Machine.Image
 		machineImageFromCloudProfile, err := determineMachineImage(cloudProfile, workerImage)
@@ -333,14 +335,17 @@ func MaintainMachineImages(logger *logrus.Entry, shoot *gardencorev1beta1.Shoot,
 		}
 
 		if !shouldBeUpdated {
-			return nil, nil, nil
+			continue
 		}
 
 		message := fmt.Sprintf("image of worker-pool '%s' from '%s' version '%s' to version '%s'. Reason: %s", worker.Name, workerImage.Name, *workerImage.Version, *updatedMachineImage.Version, *reason)
 		reasonsForUpdate = append(reasonsForUpdate, message)
 		logger.Debugf("[SHOOT MAINTENANCE] Updating %s", message)
 		shootMachineImagesForUpdate = append(shootMachineImagesForUpdate, updatedMachineImage)
 	}
+	if len(shootMachineImagesForUpdate) == 0 {
+		return nil, nil, nil
+	}
 	return shootMachineImagesForUpdate, reasonsForUpdate, nil
 }
 

pkg/controllermanager/controller/shoot/shoot_maintenance_control_test.go

@@ -172,6 +172,31 @@ var _ = Describe("Shoot Maintenance", func() {
 			Expect(workerImages[0].Version).To(PointTo(Equal(cloudProfile.Spec.MachineImages[0].Versions[1].Version)))
 		})
 
+		It("should determine that the shoot worker machine images must be maintained - multiple worker pools", func() {
+			cloudProfile.Spec.MachineImages = append(cloudProfile.Spec.MachineImages, gardencorev1beta1.MachineImage{
+				Name: "gardenlinux",
+				Versions: []gardencorev1beta1.ExpirableVersion{
+					{Version: "1.0.0"},
+				},
+			})
+
+			otherWorker := gardencorev1beta1.Worker{
+				Name: "cpu-glinux",
+				Machine: gardencorev1beta1.Machine{Image: &gardencorev1beta1.ShootMachineImage{
+					Name:    "gardenlinux",
+					Version: pointer.StringPtr("1.0.0"),
+				}},
+			}
+
+			shoot.Spec.Provider.Workers = append(shoot.Spec.Provider.Workers, otherWorker)
+			workerImages, _, err := MaintainMachineImages(testlogger, shoot, cloudProfile)
+
+			Expect(err).NotTo(HaveOccurred())
+			Expect(len(workerImages)).NotTo(Equal(0))
+			Expect(workerImages[0].Name).To(Equal(cloudProfile.Spec.MachineImages[0].Name))
+			Expect(workerImages[0].Version).To(PointTo(Equal(cloudProfile.Spec.MachineImages[0].Versions[1].Version)))
+		})
+
 		It("should update to latest non-preview version - MaintenanceAutoUpdate set to true", func() {
 			highestPreviewVersion := gardencorev1beta1.ExpirableVersion{
 				Version:        "1.1.2",