Checkmk Werk 13767: API: fix show all users endpoint call when users have no emails

ID: 13767 Title: API: fix show all users endpoint call when users have no emails Component: REST API Level: 1 Class: Bug fix Version: 2.2.0i1 In the previous version, the show all users endpoint returned an error if one of the users had no email present in its contact information. This werk fixes this behaviour. It should be noted that this case is only relevant for internal users. Users created through the REST-API or the UI will always have an email placeholder field even if an email was not specified.