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
## What are the relevant issue numbers?
#17730
See merge request !4245