fixed NPE for Already passed cron

redisson/src/main/java/org/redisson/RedissonExecutorService.java

@@ -854,10 +854,10 @@ public class RedissonExecutorService implements RScheduledExecutorService {
         byte[] classBody = getClassBody(task);
         byte[] state = encode(task);
         final Date startDate = cronSchedule.getExpression().getNextValidTimeAfter(new Date());
-        long startTime = startDate.getTime();
         if (startDate == null) {
             return null;
         }
+        long startTime = startDate.getTime();
         RemotePromise<Void> result = (RemotePromise<Void>) asyncScheduledServiceAtFixed.schedule(task.getClass().getName(), classBody, state, startTime, cronSchedule.getExpression().getCronExpression(), executorId, null);
         addListener(result);
         RedissonScheduledFuture<Void> f = new RedissonScheduledFuture<Void>(result, startTime) {