Fix type inconsistency in extract_nodes_reflexion parameter

Change group_id parameter from str = '' to str | None = None to match
the pattern used throughout the codebase and align with the optional
nature of group_id in generate_response().

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

graphiti_core/utils/maintenance/node_operations.py

@@ -65,7 +65,7 @@ async def extract_nodes_reflexion(
     previous_episodes: list[EpisodicNode],
     node_names: list[str],
     ensure_ascii: bool = False,
-    group_id: str = '',
+    group_id: str | None = None,
 ) -> list[str]:
     # Prepare context for LLM
     context = {