erp5_interface_post: fix stripMessageId() for empty string case.

4 jobs for feat/restricted_random in 0 seconds
Status Job ID Name Coverage
  External
failed ERP5.UnitTest-Master.Medusa

passed SlapOS.Eggs.UnitTest-Master.Python2

00:32:51

passed SlapOS.Eggs.UnitTest-Master.Python3

failed SlapOS.Eggs.UnitTest-Master.Python3

00:03:12