Review LogData default values

Nobody has claimed this yet.

Currently the log data returned by this library is a `LogData` structure as shown below
```
#[derive(Debug, Serialize)]
pub struct LogData {
pub subsystem: String,
pub thread_id: u64,
pub pid: u64,
pub euid: u32,
pub library: String,
pub library_uuid: String,
pub activity_id: u64,
pub parent_activity_id: u64,
pub time: f64,
pub category: String,
pub event_type: EventType,
pub log_type: LogType,
pub process: String,
pub process_uuid: String,
pub message: String,
pub raw_message: String,
pub boot_uuid: String,
pub timezone_name: String,
pub message_entries: Vec,
pub timestamp: String,
pub message_flags: Vec,
pub evidence: String,
}
```

@ReturnRei has mention a few good points that some of default values can be misleading. For example, by default `euid` is set to 0. Which on macOS is the root account.

Some of these values should be replaced with an `Option type` that is set to `None` if the value is not set when parsing the log entry
Some values that should probably be Option include
- euid
- parent_activity_id
- activity_id

Start with the LogData structure shown in the issue and trace the parser that constructs it for unset values. Review whether euid, parent_activity_id, and activity_id can be represented as None without misleading defaults, then check the affected parsing behavior and public API expectations.

Written by the indexing model from the issue text.