Fix empty import error
Fix empty import error

This fixes the case when all lines are ignored during the import
and modified_objects is None.

andrei-karalionak/ggrc-core,kr41/ggrc-core,VinnieJohns/ggrc-core,edofic/ggrc-core,AleksNeStu/ggrc-core,josthkko/ggrc-core,selahssea/ggrc-core,josthkko/ggrc-core,selahssea/ggrc-core,AleksNeStu/ggrc-core,AleksNeStu/ggrc-core,plamut/ggrc-core,VinnieJohns/ggrc-core,kr41/ggrc-core,andrei-karalionak/ggrc-core,andrei-karalionak/ggrc-core,andrei-karalionak/ggrc-core,kr41/ggrc-core,VinnieJohns/ggrc-core,selahssea/ggrc-core,j0gurt/ggrc-core,plamut/ggrc-core,josthkko/ggrc-core,josthkko/ggrc-core,AleksNeStu/ggrc-core,j0gurt/ggrc-core,selahssea/ggrc-core,edofic/ggrc-core,plamut/ggrc-core,j0gurt/ggrc-core,j0gurt/ggrc-core,kr41/ggrc-core,VinnieJohns/ggrc-core,plamut/ggrc-core,edofic/ggrc-core,edofic/ggrc-core