fix freebusy serializer

[dsegovia90]

I know this repo is autogenerated, but this change fixes empty freebusy calls.

Pre-fix response body:

Response {
    status: 200,
    headers: {
        ...
    },
    body: FreeBusyResponse {
        calendars: Some(
            FreeBusyCalendar {
                busy: [],
                errors: [],
            },
        ),
        groups: None,
        kind: "calendar#freeBusy",
        time_max: Some(
            2025-11-20T13:59:03.215Z,
        ),
        time_min: Some(
            2025-11-06T13:59:03.215Z,
        ),
    },
}

Post-fix response body

Response {
    status: 200,
    headers: {
       ...
    },
    body: FreeBusyResponse {
        calendars: {
            "somerandomid@...": FreeBusyCalendar {
                busy: [
                    TimePeriod {
                        end: 2025-11-12T21:30:00Z,
                        start: 2025-11-12T20:00:00Z,
                    },
                    TimePeriod {
                        end: 2025-11-14T21:00:00Z,
                        start: 2025-11-14T20:00:00Z,
                    },
                    TimePeriod {
                        end: 2025-11-17T17:30:00Z,
                        start: 2025-11-17T16:30:00Z,
                    },
                ],
                errors: [],
            },
            "somerandomid@...": FreeBusyCalendar {
                busy: [
                    TimePeriod {
                        end: 2025-11-06T17:00:00Z,
                        start: 2025-11-06T16:00:00Z,
                    },
                    TimePeriod {
                        end: 2025-11-07T07:45:00Z,
                        start: 2025-11-07T00:15:00Z,
                    },
                ],
                errors: [],
            },
        },
        groups: None,
        kind: "calendar#freeBusy",
        time_max: Some(
            2025-11-20T14:00:56.711Z,
        ),
        time_min: Some(
            2025-11-06T14:00:56.711Z,
        ),
    },
}