What does `foo==null && <expression>` mean?

I've found unknown for me code construction on JQuery site. After some formatting it looks like:

function (a,c) {
    c==null && (c=a,a=null);
    return arguments.length>0
        ? this.bind(b,a,c) 
        : this.trigger(b)
}

What does the first line of the function mean? Is it any trick or standard JS code construction?