DIH: fields added by update processors to $deleteDocById documents trigger warnings

<dataConfig>
  <dataSource type="FileDataSource" encoding="UTF-8"/>
  <document>
    <entity
            name="fileListDelete" processor="FileListEntityProcessor"
            baseDir="/path/to/dir" recursive="true"
            fileName="delete.xml" rootEntity="false">

      <entity
              name="xmlFileDelete" processor="XPathEntityProcessor"
              url="${fileListDelete.fileAbsolutePath}" stream="true"
              forEach="Docs/Doc">

        <field column="$deleteDocById" xpath="/Docs/Doc"/>
      </entity>
    </entity>
  </document>
</dataConfig>

<!-- solrconfig.xml -->
<updateRequestProcessorChain name="default-chain">
  <.../>

  <!-- set update timestamp -->
  <processor class="solr.DefaultValueUpdateProcessorFactory">
    <str name="fieldName">update_timestamp</str>
    <str name="value">NOW</str>
  </processor>

  <processor class="solr.DistributedUpdateProcessorFactory"/>

  <.../>
</updateRequestProcessorChain>