make first_id and last_id in ListVectorStoresResponse optional (64bit#270)

Author: 64bit

async-openai/src/types/vector_store.rs

@@ -105,8 +105,8 @@ pub struct VectorStoreFileCounts {
 pub struct ListVectorStoresResponse {
     pub object: String,
     pub data: Vec<VectorStoreObject>,
-    pub first_id: String,
-    pub last_id: String,
+    pub first_id: Option<String>,
+    pub last_id: Option<String>,
     pub has_more: bool,
 }