[views] csvexport accept an empty rset (closes #4236928)
When you tried to apply the 'csvexport' view on an empty rset, the view
couldn't be selected and you got a HTTP 500 error.
Also add two new test cases.
<html>
<head>
<!-- dependencies -->
<script type="text/javascript" src="utils.js"></script>
<script type="text/javascript" src="../../data/jquery.js"></script>
<script src="../../data/cubicweb.python.js" type="text/javascript"></script>
<script src="../../data/cubicweb.js" type="text/javascript"></script>
<script src="../../data/cubicweb.compat.js" type="text/javascript"></script>
<!-- qunit files -->
<script type="text/javascript" src="../../../devtools/data/qunit.js"></script>
<link rel="stylesheet" type="text/css" media="all" href="../../../devtools/data/qunit.css" />
<!-- test suite -->
<script src="cwmock.js" type="text/javascript"></script>
<script src="test_utils.js" type="text/javascript"></script>
</head>
<body>
<div id="main"> </div>
<h1 id="qunit-header">cw.utils functions tests</h1>
<h2 id="qunit-banner"></h2>
<ol id="qunit-tests">
</body>
</html>