fix: Modify return type of className of NavLinkProps interface

[koojaa]

It seems that there's a type collision between current NavLink interface and NavLink example demonstrated on docs.

<NavLink
    to="tasks"
    className={({ isActive }) =>
        isActive ? activeClassName : undefined
    }
>
    Tasks
</NavLink>

// Type 'string | undefined' is not assignable to type 'string'.

IMO it looks good to add undefined return type for the classname of NavLink prop instead of modifying an example.

(Existing tests passed, should any further test be required?)

[timdorr]

Can you add a test that uses undefined for the return value? That'll prevent a reversion later.

[koojaa]

@timdorr Sure 🙂

[remix-cla-bot]

Thank you for signing the Contributor License Agreement. Let's get this merged! 🥳