fix: allow "null" to Link prefetch type

[OlegLustenko]

In the same way as it default value here
https://github.com/vercel/next.js/blob/canary/packages/next/src/client/link.tsx#L279

Thoughts about this change?

What?

Adding null to prefetch type for Link component

Why?

We have some components that set prefetch=true by default,
However, in some specific cases, we want to enable "soft-prefetch".

To enable soft prefetch, we want to pass "null"

How?

By adding null to Link prefetch

[ijjk]

Allow CI Workflow Run

• approve CI run for commit: bbb019a

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer

[eps1lon]

Thank you. Could you also add a test or highlight an existing one for <Link prefetch={null} />?

I suspect it behaves just like <Link prefetch={false} /> so I'm wondering why you can't do that in your codebase? Adding type overloads for convenience usually makes code both in app code and in the implementation harder for a minor convenience in rare cases.

[OlegLustenko]

Thanks for review @eps1lon !

Yeah, I'll add a tests, there is a difference about false vs null

https://github.com/vercel/next.js/blob/canary/packages/next/src/client/link.tsx#L316
It described here

    /**
     * The possible states for prefetch are:
     * - null: this is the default "auto" mode, where we will prefetch partially if the link is in the viewport
     * - true: we will prefetch if the link is visible and prefetch the full page, not just partially
     * - false: we will not prefetch if in the viewport at all
     */
    const appPrefetchKind =
      prefetchProp === null ? PrefetchKind.AUTO : PrefetchKind.FULL

[eps1lon]

This is an implementation detail though. Why can't you use undefined instead of null?

[OlegLustenko]

It was fixed in the PR #67868

with this commit 025db7d#diff-96efb3e8013ebe2b3d6baefc46f9683fb1f7011d52d81b01be6a45cee47224d6R86

So, I'm closing this PR

Thanks for review!