Description:
------------
SolrResponse::getReponse() and SolrUtils::digestXmlResponse() are unable to parse a valid Solr xml reponse with an empty (array) value.

<str/> is not accepted
<str></str> is not accepted
<str> </str> is accepted

@see example

Reproduce code:
---------------
<?php
$s = '<?xml version="1.0" encoding="UTF-8"?>
<response>

<lst name="responseHeader">
 <int name="status">0</int>
 <int name="QTime">0</int>

 <lst name="params">
  <str name="fl">author, value</str>
  <str name="indent">on</str>
  <str name="start">0</str>
  <str name="q">myQuery</str>
  <str name="wt">xml</str>
  <str name="rows">10</str>
  <str name="version">2.2</str>
 </lst>
</lst>
<result name="response" numFound="1" start="0">
 <doc>
  <str name="author">Me</str>
  <arr name="value">
  	<str>Some text</str>
	<str/>
  </arr>
 </doc>
</result>
</response>';
$o = \SolrUtils::digestXmlResponse( $s, \SolrResponse::PARSE_SOLR_OBJ );

print_r( $o ); die;
?>

Expected result:
----------------
SolrObject Object
(
    [responseHeader] => SolrObject Object
        (
            [status] => 0
            [QTime] => 0
            [params] => SolrObject Object
                (
                    [fl] => author, value
                    [indent] => on
                    [start] => 0
                    [q] => myQuery
                    [wt] => xml
                    [rows] => 10
                    [version] => 2.2
                )

        )

    [response] => SolrObject Object
        (
            [numFound] => 1
            [start] => 0
            [docs] => Array
                (
                    [0] => SolrObject Object
                        (
                            [author] => Me
                            [value] => Array
                                (
                                    [0] => Some text
                                    [1] =>  
                                )

                        )

                )

        )

)


Actual result:
--------------
500 internal server error