default affected rows to 0

Summary:
- when no rows are affected, okPacket returns 0, not -1
- it's also not internally consistent within squangle, here it sets 0

Reviewed By: aditya-jalan

Differential Revision: D83618289

fbshipit-source-id: 242003b354d0419a7e88394a6e7b79421f6bc75b

Author: tensor-flower

third-party/squangle/src/squangle/mysql_client/DbResult.h

@@ -616,7 +616,7 @@ class StreamedQueryResult {
 
   const size_t query_idx_;
   size_t num_rows_ = 0;
-  int64_t num_affected_rows_ = -1;
+  int64_t num_affected_rows_ = 0;
   int64_t last_insert_id_ = 0;
   std::string recv_gtid_;
   RespAttrs resp_attrs_;