Fix the Bot page in the case where a task duration is None.
Fix the Bot page in the case where a task duration is None.

This happens for aborted tasks.

R=csharp@chromium.org
BUG=

Review URL: https://codereview.appspot.com/114010043

madecoste/swarming,luci/luci-py,madecoste/swarming,luci/luci-py,luci/luci-py,pombreda/swarming,madecoste/swarming,madecoste/swarming,luci/luci-py,pombreda/swarming,pombreda/swarming,pombreda/swarming