csi: account for nil volume/mount in API-to-structs conversion

Fix a nil pointer in the API struct to `nomad/structs` conversion when a
`volume` or `volume_mount` block is empty.

.changelog/10855.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+csi: Fix a bug where the HTTP server would crash if a `volume` or `volume_mount` block was empty
+```

command/agent/job_endpoint.go

@@ -932,7 +932,7 @@ func ApiTgToStructsTG(job *structs.Job, taskGroup *api.TaskGroup, tg *structs.Ta
 	if l := len(taskGroup.Volumes); l != 0 {
 		tg.Volumes = make(map[string]*structs.VolumeRequest, l)
 		for k, v := range taskGroup.Volumes {
-			if v.Type != structs.VolumeTypeHost && v.Type != structs.VolumeTypeCSI {
+			if v == nil || (v.Type != structs.VolumeTypeHost && v.Type != structs.VolumeTypeCSI) {
 				// Ignore volumes we don't understand in this iteration currently.
 				// - This is because we don't currently have a way to return errors here.
 				continue
@@ -1027,12 +1027,15 @@ func ApiTaskToStructsTask(job *structs.Job, group *structs.TaskGroup,
 
 	if l := len(apiTask.VolumeMounts); l != 0 {
 		structsTask.VolumeMounts = make([]*structs.VolumeMount, l)
-		for i, mount := range apiTask.VolumeMounts {
-			structsTask.VolumeMounts[i] = &structs.VolumeMount{
-				Volume:          *mount.Volume,
-				Destination:     *mount.Destination,
-				ReadOnly:        *mount.ReadOnly,
-				PropagationMode: *mount.PropagationMode,
+		for _, mount := range apiTask.VolumeMounts {
+			if mount != nil {
+				structsTask.VolumeMounts = append(structsTask.VolumeMounts,
+					&structs.VolumeMount{
+						Volume:          *mount.Volume,
+						Destination:     *mount.Destination,
+						ReadOnly:        *mount.ReadOnly,
+						PropagationMode: *mount.PropagationMode,
+					})
 			}
 		}
 	}