stack/erp5: Fix postfix instanciation in non-relay mode

Empty values are considered not provided, so provide a template which
produces a file with a lone comment.
6 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

03:08:44

failed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

00:45:36

passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

00:39:15

passed SlapOS.Eggs.UnitTest-Master.Python2

00:14:56

passed SlapOS.Eggs.UnitTest-Master.Python3

00:18:06

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

00:32:29