add nil check to storagetransfer_job transfer schedule (#6634) (#4745)

Signed-off-by: Modular Magician <[email protected]>

Signed-off-by: Modular Magician <[email protected]>

Author: modular-magician

.changelog/6634.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+storagetransfer: fixed a crash in `google_storagetransfer_job` refreshes when `transfer_schedule` was empty
+```

google-beta/resource_storage_transfer_job.go

@@ -839,7 +839,7 @@ func expandTransferSchedules(transferSchedules []interface{}) *storagetransfer.S
 }
 
 func flattenTransferSchedule(transferSchedule *storagetransfer.Schedule) []map[string]interface{} {
-	if reflect.DeepEqual(transferSchedule, &storagetransfer.Schedule{}) {
+	if transferSchedule == nil || reflect.DeepEqual(transferSchedule, &storagetransfer.Schedule{}) {
 		return nil
 	}