The field user_domain allow to get the list of users for which a
challenge is applied. The javascript framework for a char_domain widget
sends a JSON list to the server, but the server saved it as a string in
a Char field.
Hence we can have a list (in values received by write or create) or a
string (when getting the field value of a record).
Since 0c649644db this caused an error when user_domain was not empty.
This commit solves it by having _get_challenger_users always receiving a
string.
opw-708177
Nicolas Lempereur
83b6b71e29