Fix exporting for object level custom attributes
Fix exporting for object level custom attributes

Some assessments might not have all custom attributes and when exporting
those, we should return an empty value.

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