Fix since time exit display when s.FinishedAt is zero

When s.FinishedAt is zero, the since time exit in docker ps doesn't display correct time.
For example
```
Exited (0) 292.471209 years ago
```
This patch fixes the since time exit to display nothing if s.FinishedAt is zero.

Docker-DCO-1.1-Signed-off-by: Ken ICHIKAWA <ichikawa.ken@jp.fujitsu.com> (github: ichik1)