elasticsearchwriter 的字段null问题

eswriter 处理text类型时在mysql中字段值是null的，也应该是返回null，但是同步到es中是不显示null请问怎么解决呢？
除了text类型之外，其他类型也是如此。

Start by tracing the eswriter path that handles MySQL NULL values for text and other field types, then inspect how values are serialized for Elasticsearch. Reproduce the behavior with NULL inputs across the mentioned types and define done as preserving the intended NULL representation during synchronization, with regression coverage for each affected type.

Written by the indexing model from the issue text.