Don't perform a scenario computation when no gmvs are available

This is to avoid baffling error messages. When an empty array is passed, numpy.mean returns a NaN, which is not an array and gives a bad error message in a far away region of the code (in the writer of the scenario_damage calculator). The gmvs array can be empty in a QA test if for instance the data have not been restored due to
a conflict of ids.