[test] fix req default value handling in CubicWebTC.view (closes #2544376)
It's no more than the standard ``if rset`` vs. ``if rset is None`` error.
<div id="ajaxroot">
<div class="ajaxHtmlHead">
<script src="http://foo.js" type="text/javascript"> </script>
<link rel="stylesheet" type="text/css" media="all" href="qunit.css" />
</div>
<h1>Hello</h1>
</div>