None values in list fields bound to users are not well handled and produce some errors :
[warning] [XML import][art # 931] Skipped invalid value 0 for field assigned_to
[warning] [XML import][art # 931] Skipped invalid value 0 for field assigned_to
[warning] [XML import][art # 932] Skipped invalid value 0 for field assigned_to
[warning] [XML import][art # 5219] Skipped invalid value 0 for field assigned_to
[warning] [XML import][art # 5228] Skipped invalid value 0 for field assigned_to
[warning] [XML import][art # 5237] Skipped invalid value 0 for field assigned_to
[warning] [XML import][art # 5238] Skipped invalid value 0 for field assigned_to
[warning] [XML import][art # 5258] Skipped invalid value 0 for field assigned_to
[warning] [XML import][art # 5259] Skipped invalid value 0 for field assigned_to