Fix EZP-23497: allow user de-selection when editing multiplexer workflow.
Fix EZP-23527: ezdatetime with no default value causes class edit to fail

According to the other datatypes I have checked, it should return an empty array in this case.
Merge pull request #1101 from glye/ezp-23527_ezdatetime_class_edit_fail

Fix EZP-23527: ezdatetime with no default value causes class edit to fai...
Merge pull request #1095 from joaoinacio/EZP-23497_multiplexer_edit_users

Fix EZP-23497: allow user de-selection when editing multiplexer workflow