Fix step None bug

Step text might be None sometimes when the step is failed.
It broke the build detail page. Check the screenshot attached.
https://trello-attachments.s3.amazonaws.com/520e20a062ccc3b70d0029d0/52a1ca4124eaaa752a005976/21abdc7f2b56df6af1244653e3bbd0e1/build_detail_bug.png
Merge smokingkapor/buildbot:master (PR #1061)