Log only when Zombies exists (#20118)
Log only when Zombies exists (#20118)

The following feels like there were non-zero jobs that were zombies while there weren't
```
[2021-12-07 20:54:35,502] {manager.py:1051} INFO - Finding 'running' jobs without a recent heartbeat
[2021-12-07 20:54:35,502] {manager.py:1055} INFO - Failing jobs without heartbeat after 2021-12-07 20:49:35.502841+00:00
[2021-12-07 20:54:45,541] {manager.py:1051} INFO - Finding 'running' jobs without a recent heartbeat
[2021-12-07 20:54:45,542] {manager.py:1055} INFO - Failing jobs without heartbeat after 2021-12-07 20:49:45.542170+00:00
```

This changes so that logs only exists when we have more than 0 zombies
mistercrunch/airflow,nathanielvarona/airflow,nathanielvarona/airflow,cfei18/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,apache/airflow,bolkedebruin/airflow,nathanielvarona/airflow,mistercrunch/airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,cfei18/incubator-airflow,bolkedebruin/airflow,apache/airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,apache/airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,lyft/incubator-airflow,lyft/incubator-airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,lyft/incubator-airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,apache/airflow