erp5_configurator: prevent zerodivisionerror

After discussing on
!1103 (comment 104474) we
amended the original commit so that if there are no bt to install at
this point we consider this step is finished (use 1 and not 0)
3 jobs for arnau in 0 seconds
Status Job ID Name Coverage
  External
failed ERP5.UnitTest-TestRunner2

08:56:17

failed ERP5.UnitTest-TestRunner2

08:27:20

failed ERP5.UnitTest-TestRunner2

28:12:14