title: Redshift FULL OUTER JOIN doesn't output NULL
body: <p>We have a 'numbers' table that holds 0-10000 values in its single value 'n'.
  We have tableX that has calculated_at datetime and a term.
  We are trying to fill the holes where in tableX doesnt have matches in the given dates. HOWEVER, this doesn't seem to yield NULL or 0 for the non-matching... </p>
  
  <pre><code>select term
       , avg(total::float)
       , date_trunc('day', series.date) as date1
       , date_trunc('day', calculated_at) as date2
  from (select 
          (current_timestamp - interval '1 day' * numbers.n)::date as date
        from numbers) as series
  full outer join terms
     on series.date = date_trunc('day', calculated_at)
  where series.date BETWEEN '2017-07-01' AND '2017-07-30'
  AND (term in ('term111') or term is null)
  group by term
         , date_trunc('day', series.date)
         , date_trunc('day', calculated_at)
   order by date_trunc('day', series.date) asc
  </code></pre>