TypeScript: Use generics to access property if object is not null

AccessPropIfNotNull.ts

class Person {
    firstname: string;
    lastname: string;
    constructor(firstname: string, lastname: string) {
        this.firstname = firstname;
        this.lastname = lastname;
    }    
}

let p = new Person('Sudhanshu', 'Mishra');
let q: Person;
function accessPropIfNotNull<T>(arg: T, prop: string): T {
    if (arg != null && arg.hasOwnProperty(prop)) {
        return arg[prop];
    }

    return null;
}

console.log(accessPropIfNotNull(p, 'firstname')); // returns firstname
console.log(accessPropIfNotNull(q, 'firstname')); // returns null