title: ABAP CDS view entities expression type IS_NULL not allowed in the BOOLEAN expression context
body: <p>I would like to use the is null expression in where clause of a CDS view entity. It says that expression type IS_NULL is not allowed in the BOOLEAN expression context.</p>
  <p>I joined (left join) a table and want to assure that the joined result/fields are null, so no match.</p>
  <p>In CDS entities the same can be applied. The fields of the joined table with no match are null and can be checked in the where clause.</p>
  <pre><code>define view entity ZI_EntityExample
    as select from    aufk
      join            afko as orderheader on orderheader.aufnr = aufk.aufnr
      left outer join jest as status      on  status.objnr = aufk.objnr
                                          and status.stat  = 'I0009'
      {
        key aufk.aufnr as OrderId,
            aufk.objnr as ObjectId
      }
      where
        status.stat is null
  </code></pre>
  <p>As shown in the example I would like to select out the confirmed orders via join or association.</p>