Description:
------------
Exception is generated When result contain empty doc element.

Response from server is like this: 

<?xml version="1.0" encoding="UTF-8"?>
<response>

<result name="response" numFound="167" start="0">
  <doc>
    <int name="ProductCategoryId">18918</int>
    <int name="ProductId">19646</int></doc>
  <doc>
    <int name="ProductCategoryId">18918</int>
    <int name="ProductId">19642</int></doc>
<doc>
    <int name="ProductCategoryId">18920</int>
    <int name="ProductId">18190</int></doc>
  <doc></doc>
</result>
</response>

Empty "doc" element is returned by Solr because found document has no field requested by query.

Test script:
---------------
$qResponse = $client->query($query);
$qResponse->setParseMode(\SolrQueryResponse::PARSE_SOLR_DOC);
$response = $qResponse->getResponse(); //here is the error