erp5_document_scanner: prevent passing empty string as device_id

5 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

01:45:15

passed ERP5.PerformanceTest-Master

00:32:24

passed ERP5.UnitTest-Master

03:08:23

passed SlapOS.Eggs.UnitTest-Master.Python2

00:43:20

passed SlapOS.Eggs.UnitTest-Master.Python3

00:35:02