title: How to remove empty row in BQ
body: <p>I'm just trying to solve simple examples to improve my skill on writing queries in BigQuery. In the below example, 
  I don't know how to remove the empty row.</p>
  
  <pre><code>with table1 as(
  select "box_1" box, "yellow" colours union all
  select "box_1" box, "green" colours union all
  select "box_2" box, "blue" colours union all
  select "box_2" box, "blue" colours union all
  select "box_3" box, "red" colours union all
  select "box_3" box, "green" colours union all
  select "box_3" box, "blue" colours
  )
  select array(select box from unnest(x)y where "blue" in unnest(x) limit 1 )box_containing_blue 
  from(select box,array_agg(if(colours="blue",colours,null)ignore nulls )x 
  from table1 group by box)
  </code></pre>