Fix case when the pidfile is empty during the pidfile check

[potiuk]

Bug was introduced with commit that has not been cherry-picked to 1.10: 77bc480

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[kaxil]

Worth including in 1.10.11 ? Is there a related Bug/Issue that exists? If so, would be great to add details (PR Description) or link to the issue. If not, doesn't matter

[potiuk]

It's 2.0 thing only I think. I believe It was introduced with commit that has not been cherry-picked to 1.10: 77bc480
Author: Tomek Urbaszek [email protected]
Date: Mon Mar 23 22:32:23 2020 +0100

[AIRFLOW-6624] Improve webserver command with pidfile checking (#7245)

[kaxil]

It's 2.0 thing only I think. I believe It was introduced with commit that has not been cherry-picked to 1.10: 77bc480
Author: Tomek Urbaszek [email protected]
Date: Mon Mar 23 22:32:23 2020 +0100

[AIRFLOW-6624] Improve webserver command with pidfile checking (#7245)

Got it 👍

[kaxil]

It's 2.0 thing only I think. I believe It was introduced with commit that has not been cherry-picked to 1.10: 77bc480
Author: Tomek Urbaszek [email protected]
Date: Mon Mar 23 22:32:23 2020 +0100
[AIRFLOW-6624] Improve webserver command with pidfile checking (#7245)

Got it 👍

Added description to the PR