Fix workflow imports with empty date fields
Fix workflow imports with empty date fields

Empty fields in imports automatically mean that the value is ignored and
should not be changed on updates.

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