Github user wangyum commented on the issue: https://github.com/apache/spark/pull/23164 I think we should back-port because we can [`putNull`](https://github.com/apache/spark/blob/630e25e35506c02a0b1e202ef82b1b0f69e50966/sql/catalyst/src/main/scala/org/apache/spark/sql/types/Metadata.scala#L246), but can not serialize it without this patch.
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