kola: fix type assertion on possibly nil interface
kola: fix type assertion on possibly nil interface

quantum/coreos-mantle,quantum/coreos-mantle,glevand/coreos--mantle,mjg59/mantle,dm0-/mantle,mjg59/mantle,dm0-/mantle,marineam/mantle,glevand/coreos--mantle,marineam/mantle