title: Wrong query evaluation with aggregation subquery
body: <p>Please refer to <a href="http://factforge.net/sparql?name=&amp;infer=false&amp;sameAs=false&amp;query=SELECT+%0A%3FmyVar+%3Fp+%3Fo%0AWHERE+%0A%7B+++%0A++%7B+%0A++++SELECT+%3FmyVar+%0A++++++++WHERE+%7B+%0A++++++++++++%3FmyVar+%3Chttp%3A%2F%2Fwww.example.com%2FarbitraryNonExistent%3E+%3Fxx.+%0A++++++++%7D+%0A++++GROUP+BY+%3FmyVar%0A++%7D+%0A++%3FmyVar+%3Fp+%3Fo.++%0A%7D+%0ALIMIT+10" rel="nofollow noreferrer">Factforge Endpoint</a> to execute this query. The subquery doesn't return any results. <code>?myVar</code> will be projected out to the containing query, and then joined with the triple pattern <code>?myVar ?p ?o.</code>. But as there are no results from the inner select, the join should result in nothing. However, this is not the case when executing the query. Isn't this a bug?</p>
  
  <pre><code>SELECT 
  ?myVar ?p ?o
  WHERE 
  {   
    { 
      SELECT ?myVar 
          WHERE { 
              ?myVar &lt;http://www.example.com/arbitraryNonExistent&gt; ?xx. 
          } 
      GROUP BY ?myVar
    } 
    ?myVar ?p ?o.  
  } 
  LIMIT 10
  </code></pre>