Bugfixed for 'null' type of msgCallback value to avoid stop rendering from the chart.

[EdHuSplunk]

Bugs: The client side (browser) will throw an error and stop rendering the chart if the parameter of msgCallback function is 'null' or other non-object type of data, then it will throw the error since there is no 'type' attribute that "msg" can be accessed.

Bugfixed for 'null' type of msgCallback value to avoid stop to render on the chart by adding a type checker function:

    if(!msg) return;