erp5_hal_json_style: add compatibility with URL columns returning None value

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passed ERP5.CodingStyleTest-Master

01:03:50

failed ERP5.UnitTest-Master

01:56:33

failed ERP5.UnitTest-Master.Medusa

01:59:44

passed SlapOS.Eggs.UnitTest-Master.Python2

00:39:09

passed SlapOS.Eggs.UnitTest-Master.Python3

00:13:17