GTTF: Handle case where JSON results are not generated.
GTTF: Handle case where JSON results are not generated.

One example when it happens is unit test crash.

BUG=317931

Review URL: https://codereview.chromium.org/113913006

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@242044 0039d316-1c4b-4281-b951-d872f2087c98

luci/recipes-py,shishkander/recipes-py,luci/recipes-py,shishkander/recipes-py