feat!: add null to return type of serialize()

[CatBraaain]

Allow the serialize() function to return null to support clearing the query parameter, similar to useQueryState.

Background

In useQueryState, null consistently represents the absence of a query parameter:

• When reading, the returned value is null if the key is not present in the URL.
• When writing, passing null to the setter removes the key from the query string.

ref:

• https://nuqs.dev/docs/basic-usage
• https://nuqs.dev/docs/batching#options

Example

This change enables parsers to conditionally omit values from the URL. For example, a parser could return null for certain values to keep
the URL clean:

const parser = createParser({
  parse: (value) => parseInt(value),
  serialize: (value) => value === 0 ? null : String(value)
})

When 0 is set, the parameter will be cleared from the URL instead of showing ?key=0.

[vercel]

@CatBraaain is attempting to deploy a commit to the 47ng Team on Vercel.

A member of the Team first needs to authorize it.

[franky47]

This seems like a useful addition, but the widening of the return type for serialize is a breaking change (see the failure in the docs build).

I'll keep this one here for the v3 release line.

[CatBraaain]

Thanks for taking a look.
I see about the breaking change, that makes sense.

I've pushed new commit fixes the type errors and formatting issues.