Multiple applications of `restore_attrs` HTTP API calls set attribute to a null value

Describe the bug

Bam, Oida

# curl -k -s -u "username:password" https://localhost:5665/v1/objects/services \
    | jq '.results | .[] |  select(.attrs.__name == "test!Test2") | .attrs.max_check_attempts'
3
# curl -k -s -u "username:password" https://localhost:5665/v1/objects/services  -X POST -H 'Accept: application/json' \
    -d '{ "attrs": {"max_check_attempts": 7}}'
{"results":[{"code":200,"name":"test!Test2","status":"Attributes updated.","type":"Service"}]}
# curl -k -s -u "username:password" https://localhost:5665/v1/objects/services \
    | jq '.results | .[] |  select(.attrs.__name == "test!Test2") | .attrs.max_check_attempts'
7
# curl -k -s -u "username:password" https://localhost:5665/v1/objects/services  -X POST -H 'Accept: application/json' \
    -d '{ "restore_attrs": ["max_check_attempts"]}' | jq .
{
  "results": [
    {
      "code": 200,
      "name": "test!Test2",
      "status": "Attributes updated.",
      "type": "Service"
    }
  ]
}
# curl -k -s -u "username:password" https://localhost:5665/v1/objects/services \
    | jq '.results | .[] |  select(.attrs.__name == "test!Test2") | .attrs.max_check_attempts'
3
# curl -k -s -u "username:password" https://localhost:5665/v1/objects/services  -X POST -H 'Accept: application/json' \
    -d '{ "restore_attrs": ["max_check_attempts"]}' | jq .
{
  "results": [
    {
      "code": 200,
      "name": "test!Test2",
      "status": "Attributes updated.",
      "type": "Service"                                                                                                                                                                                      }
  ]
}
# curl -k -s -u "username:password" https://localhost:5665/v1/objects/services \
    | jq '.results | .[] |  select(.attrs.__name == "test!Test2") | .attrs.max_check_attempts'
0

Expected behavior

An attribute should not be set to the null value, but not be modified at all after restoring the original value.

Your Environment

• Version used (icinga2 --version): 2.16.5-1

Start with the HTTP API handling POST requests to /v1/objects/services and the restore_attrs behavior shown in the curl reproduction. Repeat the two restore requests and verify that max_check_attempts remains 3 rather than becoming 0 or null; the fix is done when restoring repeatedly leaves the original value unchanged.