REST API groupuserpicker with empty query parameter behavior.

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.

P.S.:
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.

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