sphinx doesn't seem to support relative paths for logos anymore
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)