ServerFn started converting null to undefined in RC version

[jakst]

Which project does this relate to?

Start

Describe the bug

When sending just the value null to a serverFn, without wrapping it in an object, it is converted to undefined once it reaches the handler.

This is a regression in the RC version of Tanstack Start.

Your Example Website or App

https://github.com/jakst/tanstack-start-null-data-serverfn

Steps to Reproduce the Bug or Issue

1. Install deps with pnpm install and start dev server with pnpm dev
2. Open http://localhost:3000
3. Click the button that says Call serverFn with data = null
4. Observe the error message. The serverFn received data = undefined instead of data = null

Expected behavior

I expect null values to be preserved in the serverFn.

Screenshots or Videos

No response

Platform

• Router / Start Version: 1.132.51
• OS: [e.g. macOS, Windows, Linux]
• Browser: [e.g. Chrome, Safari, Firefox]
• Browser Version: [e.g. 91.1]
• Bundler: vite
• Bundler Version: 7.1.9

Additional context

No response