Updating a group through the REST API resets the fields that are not passed in the JSON object

For instance, the parent groups are reset il updating a group without passing the "parentGroups" property:

1. Create a "parent" group

   http -a Administrator:Administrator POST http://localhost:8080/nuxeo/api/v1/groups entity-type=group id=parent
   

2. Create a "child" group with "parent" as a parent group

   http -a Administrator:Administrator POST http://localhost:8080/nuxeo/api/v1/groups entity-type=group id=child parentGroups:='["parent"]'
   

3. Verify the parent/child relation

   http -a Administrator:Administrator GET http://localhost:8080/nuxeo/api/v1/groups/child?fetch.group=parentGroups
   
   {
       "entity-type": "group", 
       "grouplabel": "child", 
       "groupname": "child", 
       "id": "child", 
       "parentGroups": [
           "parent"
       ], 
       "properties": {
           "description": null, 
           "grouplabel": null, 
           "groupname": "child", 
           "tenantId": null
       }
   }
   

4. Update the "child" group to add the "Administrator" user a a member

   http -a Administrator:Administrator PUT http://localhost:8080/nuxeo/api/v1/groups/child entity-type=group id=child memberUsers:='["Administrator"]'
   

5. Fetch the group with is member users and parent groups

   http -a Administrator:Administrator GET http://localhost:8080/nuxeo/api/v1/groups/child?fetch.group=memberUsers,parentGroups
   
   {
       "entity-type": "group", 
       "grouplabel": "child", 
       "groupname": "child", 
       "id": "child", 
       "memberUsers": [
           "Administrator"
       ], 
       "parentGroups": [], 
       "properties": {
           "description": null, 
           "grouplabel": null, 
           "groupname": "child", 
           "tenantId": null
       }
   }
   

   The parent groups have been reset.

See recent changes made for NXP-22542, but the issue was probably already there.