User without RFID cannot be updated

A user can not be updated without having an RFID set.

Error message «empty values not allowed» is shown.

removed To Do label

has to be checked together with the web-ui-form. This could be even a javascript bug, for some reason the updating of the type in the jsonschema from string to ['null', 'string'] does not work, but only if the key is rfid.

changed milestone to %Coding Weekend HS19