Fix Error 500 in CI charts by gracefully handling commits with no durations
What does this MR do?
In the CI charts, this MR reports the duration of a commit to 0 if it is nil.
Are there points in the code the reviewer needs to double check?
Should we omit this commit from the chart or set it to some other value?
Why was this MR needed?
We were getting an Error 500 here: https://gitlab.com/gitlab-org/gitlab-ce/graphs/master/ci