title: Big query update struct to null in repeated field
body: <p>In Google bigquery, I'm trying to do an update on a repeated field.</p>
  
  <p>For comparison, this works (or at least is flagged as valid), but of course isn't actually updating the field.</p>
  
  <pre><code>UPDATE my.table t
  SET my_field = ARRAY(
    SELECT AS STRUCT g.foo, g.bar, g.struct_to_set_null
    FROM unnest(t.groups) as g
  ), ... FROM ... etc
  </code></pre>
  
  <p>Setting <code>struct_to_set_null</code> to null gives an error: </p>
  
  <pre><code>UPDATE my.table t
  SET my_field = ARRAY(
    SELECT AS STRUCT g.foo, g.bar, null as struct_to_set_null
    FROM unnest(t.groups) as g
  ), ... FROM ... etc
  </code></pre>
  
  <blockquote>
    <p><code>Value of type ARRAY&lt;STRUCT&lt;... (really long and cut off) cannot be assigned to groups, which has type &lt;ARRAY,STRUCT&lt;... (same, really long, cut off)</code></p>
  </blockquote>
  
  <p>I can see that the field in question is of type <code>RECORD</code> and <code>NULLABLE</code>, so I would think setting it to <code>null</code> is allowed. Is there a trick to getting this to work?</p>