fix default jetcontext when no GeoModel flags exist

The latest change in Jet context induced an issue in TRUTH derivation : https://its.cern.ch/jira/browse/ATLJETMET-1723

This MR fixes the problem by detecting when flags.GeoModel.Run is not defined in the job and returning an empty context in this case.

I didn't find a better way to test for this existance than with a try...except block. In particular flags.hasFlag("GeoModel.Run") always seem to return false even in case where it's actually valid... I'm happy to follow suggestions if any is proposed.