Accept empty MongoDB trace batches without inserting

great_ai/persistence/mongodb_driver.py

@@ -75,6 +75,8 @@ class MongoDbDriver(TracingDatabaseDriver):
             return client[self.mongo_database].traces.insert_one(serialized).inserted_id
 
     def save_batch(self, documents: List[Trace]) -> List[str]:
+        if not documents:
+            return []
         serialized = [d.to_flat_dict() for d in documents]
         for s in serialized:
             s["_id"] = s["trace_id"]

tests/test_mongodb_queries.py

@@ -38,3 +38,9 @@ def test_zero_take_preserves_count_without_fetching(
     driver, collection = mongo
     assert driver.query(take=0) == ([], 3)
     collection.find.assert_not_called()
+
+
+def test_empty_batch_is_a_noop(mongo: Tuple[MongoDbDriver, Any]) -> None:
+    driver, collection = mongo
+    assert driver.save_batch([]) == []
+    collection.insert_many.assert_not_called()