config_service: Fixed bug regarding None entries in latest_revisions dict.
config_service: Fixed bug regarding None entries in latest_revisions dict.

Using the include_files option on config sets that had no latest revision
number caused a 503 Internal Server Error because there was no check for
None entries in the result from get_latest_revisions_async. This cl fixes
that by adding a check.

BUG=730832

Review-Url: https://codereview.chromium.org/2981473002

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