FiveBeansWorker work functions: treat a falsy action as success

This change allows functions that follow standard node conventions to be used as the callback in worker functions.

For example, instead of this:

    Handler.prototype.work = function(payload, callback) {
      sendMail({...}, function(err) {
        if (err) return callback(err);
        else return callback('success');
      })
    };

you can now write:

    Handler.prototype.work = function(payload, callback) {
      sendMail({...}, callback);
    };

or possibly even:

    Handler.prototype.work = sendMail;