Replace duration==None to 0 when saving to XML.
Replace duration==None to 0 when saving to XML.

It can happen when a test case is systematically crashing, so that no duration
is calculated.

TBR=csharp@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/13042023

git-svn-id: d5a9b8648c52d490875de6588c8ee7ca688f9ed1@191008 0039d316-1c4b-4281-b951-d872f2087c98

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py