fix(logger): handle empty/corrupt member log files gracefully

Return None instead of crashing when check_previous_roles finds no
parsable JSON lines in a member's log file.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/advanced_logger.py

@@ -140,6 +140,10 @@ class AdvancedLogger:
                         except json.JSONDecodeError:
                             continue # Skip corrupt lines
 
+                # Guard against empty logs list after parsing
+                if not logs:
+                    return None
+
             # Find the most recent leave event
             leave_events = [log for log in logs if isinstance(log, dict) and log.get("event_type") == "member_leave"]
             if leave_events: