DAG client thinks app is still running when app status is null

When an RM restarts without recovering apps (i.e.: either work-preserving is not enabled or state store was removed) and the YARN application history is enabled then YarnClient can return an application report with the app status as null. The RM doesn't know about the application, so the client redirects to the AHS. The AHS knows the app started at some point but will never received a finished event, hence the null app status.

The DAG client fails to detect this scenario and believes the app is still running, so for example Hive clients will continue to hammer for status on an app that doesn't exist.