According to documentation action.update method has only one mandatory property "actionid" and other properties are optional.
Doing such update

{
    "actionid": "77"
}

produces an error from DB:
Cannot perform update statement on table "actions" without values.

Broken by:
krists [rev.47383]: A.F.....S. [ZBXNEXT-1133] Add support for custom expressions in action conditions.