R1:01040e457307

Correctly disinguish between "0 seconds behind master" and "not replicating"

Summary: Fixes T11159. We get two different values here (`NULL` and `0`) with different meanings.

Test Plan:
- Ran `STOP SLAVE;`.
- Saw this:

{F1710181}

- Ran `START SLAVE;`.
- Back to normal.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11159

Differential Revision: https://secure.phabricator.com/D16225