Checks to see if passed in argument is empty.
@function module:sjl.empty
@param value {*} - Value to check.
@returns {Boolean}
function isEmpty(value) {
        var classOfValue = classOf(value),
            retVal;

        // If value is an array or a string
        if (classOfValue === _Array || classOfValue === _String) {
            retVal = value.length === 0;
        }

        else if ((classOfValue === _Number && value !== 0) || (classOfValue === _Function)) {
            retVal = false;
        }

        else if (classOfValue === _Object) {
            retVal = isEmptyObj(value);
        }

        // If value is `0`, `false`, or is not set (!isset) then `value` is empty.
        else {
            retVal = !isset(value) || value === 0 || value === false;
        }

        return retVal;
    }