REST API endpoint
/rest/api/3/groupuserpicker with parameter query. In the documentation, there is no information about the empty query parameter.
In practice, if you request data from URL
/rest/api/3/groupuserpicker?query= you will get information about groups, but information about users will be empty.
On the other hand, if you request data from URL
/rest/api/3/groupuserpicker?query=%00 you will get information about users, but not groups.
The main question is this behavior correct? Or it can change in the future.
Some suspicion details. Let look at selectors for the single user/group custom field.
Let pass empty query:
- user selector will return empty search list
- group selector will return all (limited to 20) groups
Their behavior is different. The result from the user selector is suspicion.