Only update storage if there was something to insert

Before, the `finally` block would always call `_insert_done()`, which writes out the `vdb_*` and `kv_store_*` files ... even if there was nothing to insert (because all docs had already been inserted).  This was causing the speed of skippable inserts to become very slow as the graph grew.

lightrag/lightrag.py

@@ -227,6 +227,7 @@ class LightRAG:
         return loop.run_until_complete(self.ainsert(string_or_strings))
 
     async def ainsert(self, string_or_strings):
+        update_storage = False
         try:
             if isinstance(string_or_strings, str):
                 string_or_strings = [string_or_strings]
@@ -240,6 +241,7 @@ class LightRAG:
             if not len(new_docs):
                 logger.warning("All docs are already in the storage")
                 return
+            update_storage = True
             logger.info(f"[New Docs] inserting {len(new_docs)} docs")
 
             inserting_chunks = {}
@@ -286,6 +288,7 @@ class LightRAG:
             await self.full_docs.upsert(new_docs)
             await self.text_chunks.upsert(inserting_chunks)
         finally:
+            if update_storage:
                 await self._insert_done()
 
     async def _insert_done(self):