[BUG]JSON.toJSON和JSON.toJSONString对值为null的属性处理逻辑不一致

### 问题描述

使用默认feature时，JSON.toJSON和JSON.toJSONString对值为null的属性处理逻辑不一致，
JSON.toJSON保留了值为null的属性，JSON.toJSONString则忽略了null值属性。

### 期待的正确结果

JSON.toJSON默认也忽略null值属性，除非显式指明JSONWriter.Feature.WriteNulls。

Start at the JSON.toJSON and JSON.toJSONString entry points mentioned in the issue, then reproduce their handling of null-valued properties with default features and with JSONWriter.Feature.WriteNulls. Add a regression test showing that the default JSON.toJSON result omits null properties while WriteNulls preserves them.

Written by the indexing model from the issue text.