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