处理字符串原型中的null类型

String.prototype.capitalize = function() {
    return this.charAt(0).toUpperCase() + this.slice(1);
}

"".capitalize must return ""
undefined.capitalize must return ""
null.capitalize must return ""

String.prototype.capitalize = function() {
    return this.charAt(0).toUpperCase() + this.slice(1) || "";
}

Uncaught TypeError: Cannot read property 'capitalize' of undefined at <anonymous>:1:6