Allow `xmlns' attribute in XML literals

`<x xmlns=""/>` is desugared to:

    private[this] val res0: scala.xml.Elem = {
      var $tmpscope: scala.xml.NamespaceBinding = scala.this.Predef.$scope;
      $tmpscope = new scala.xml.NamespaceBinding(null, null, $tmpscope);
      {
        val $scope: scala.xml.NamespaceBinding = $tmpscope;
        new scala.xml.Elem(null, "x", scala.xml.Null, $scope, true)
      }
    };

So not only we need to ignore `null` parameters of `scala.xml.Elem` constructor, but also of `scala.xml.NamespaceBinding`.