erp5_web: fixup: QUERY_STRING is not always defined

See bcd82831 (comment 216152)
12 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed ERP5.CodingStyleTest-Master

02:28:45

failed ERP5.PerformanceTest-Master

00:35:30

failed ERP5.UnitTest-Master

08:10:29

passed SlapOS.Eggs.UnitTest-Master.Python2

00:39:44

passed SlapOS.Eggs.UnitTest-Master.Python3

00:38:54

passed Wendelin.UnitTest-Master

00:43:59

failed ERP5.CodingStyleTest-Master

01:15:05

failed ERP5.PerformanceTest-Master

00:35:31

failed ERP5.UnitTest-Master

03:18:03

passed SlapOS.Eggs.UnitTest-Master.Python2

00:25:04

passed SlapOS.Eggs.UnitTest-Master.Python3

00:27:43

passed Wendelin.UnitTest-Master

00:36:58