The start order API throws an error when the START_TIME is not specified

Current Situation

• The JOC Cockpit throws the error: "formats 'now', 'now + HH:mm:[ss]' or 'now + SECONDS' expected for \"Start time\": null" when a user calls the API ./orders/start without specifying the "at" (START_TIME).

• The JOC API documentation includes that the option "at" is optional and that by default it uses the value "now"

Desired Behavior

• The ./orders/start API should use the default value for "at" instead of throwing the error.