How to use the assert.isNull function in assert

"Converting a valid KeyboardEvent object that would result in an invalid Combo returns null": function() {
                // Simulated KeyboardEvent
                var ev = {
                    which:    null,
                    shiftKey: false,
                    altKey:   false,
                    metaKey:  false,
                    ctrlKey:  false
                };

                var combo = Combo.fromEvent(ev);
                assert.isNull(combo);
            },
            "Converting a null or invalid KeyboardEvent object returns null": function() {

return function testIO(err, output) {
        assert.isNull(err); // no errors
        assert.strictEqual(output, expected); // last actual ouput equals expected
    };
}

function assertCorrectResponse(err,data){
   assert.isNull(err);
   assert.isObject(data);
   assert.equal(data.responseHeader.status,0);
   assert.isObject(data.moreLikeThis);
   assert.equal(data.responseHeader.params['mlt.fl'],'title_t,description_t');
   assert.equal(data.responseHeader.params['mlt.mintf'],'1');
   assert.equal(data.responseHeader.params['mlt.mindf'],'1');
   assert.equal(data.responseHeader.params['mlt.minwl'],'3');
   assert.equal(data.responseHeader.params['mlt.maxwl'],'5');
   assert.equal(data.responseHeader.params['mlt.maxqt'],'8');
   assert.equal(data.responseHeader.params['mlt.maxntp'],'9');
   assert.equal(data.responseHeader.params['mlt.boost'],'true');
   assert.equal(data.responseHeader.params['mlt.qf'],'title_t^2 description_t^3');
   assert.equal(data.responseHeader.params['mlt'],'true');
   assert.equal(data.responseHeader.params['mlt.count'],'10'); 
}

'null': function (topic) {
                var null_value = topic.get_icon();
                assert.isNull(null_value);
            }
        },

'the process pid should be null': function (error, monitor) {
        assert.ifError(error);
        assert.isNull(monitor.pid.process);
      }
    },

'should return null after the timeout': function (err, nil) {
                assert.isNull(nil);
            },

'success' : function( err, message ){
          assert.isNull( err );
          assert.deepEqual( message, params );
        },
        'close connection' : {

"deserializing an empty JSON string returns null": function() {
                var result = Combo.deserialize('');
                assert.isNull(result);
            },
            "deserializing a non-Combo object using Combo.deserialize throws an Error": function() {

"outer": function(tbl) {
      var report = {};
      var song = tbl.one(
        { title: {like$: "アルメ"}
        },
        { explain: report,
          join: { tag : { via : "song_tag", as : "tags", outer: true, word: {like$: "cccc"} } }
        }
      );
      assert.isNull(song.tags);
    },
   },

"a dir with a reduced number of files": function (error,files) {
                assert.isNull(error);
                assert.equal(files.length,0);
            }
        }