Avoid setting AuditSystemState::Starting when target list is empty.
Now checks target availability before state transition, keeping the
system in Stopped state if no enabled targets are found.
- Check targets.is_empty() before setting Starting state
- Return early with Ok(()) when no targets exist
- Maintain consistent state machine behavior
- Prevent transient "Starting" state with no actual targets
Resolves issue where audit system would incorrectly enter Starting
state even when configuration contained no enabled targets.
