title: Big Query Dataflow Load to NULL partition
body: <p>Using the apache beam Python SDK, I've set up a dataflow that writes to the individual partitions in a date partitioned table on Big Query. According to <a href="https://cloud.google.com/bigquery/docs/partitioned-tables#comparing_partitioning_options" rel="nofollow noreferrer">this documentation</a> every date partitioned table has special <code>NULL</code> and <code>UNPARTITIONED</code> partitions. According to those docs I can write to the <code>UNPARTITIONED</code> partition by just setting my date far in the past or future, but how can I write to the <code>NULL</code> partition? </p>
  
  <p>I'm trying to load data to a partition based off values in the data and sometimes the field is null. I'd rather write to the <code>NULL</code> partition than make up a date to use for nulls. </p>
  
  <p>For reference, I write to date partitions doing something like this:</p>
  
  <pre><code>beam.io.Write(beam.io.BigQuerySink(table_id+'$20180925',
      project=project_id, dataset=dataset_id, schema=schema))
  </code></pre>
  
  <p>What do I need to replace <code>$20180925</code> with to write to <code>NULL</code>? </p>