Unexpected behavior when getServerSideProps is defined but is set to undefined

Run next info (available from version 12.0.8 and up)

No response

What version of Next.js are you using?

12.0.7

What version of Node.js are you using?

16.3.0

What browser are you using?

Firefox

What operating system are you using?

macOS

How are you deploying your application?

next start

Describe the Bug

When getServerSideProps is defined but is set to undefined next returns html on data request. Undefined is obviously invalid value but it is easy to happen if the page uses for example a higher-order function to wrap the getServerSideProps and it can be very confusing to find the cause.

Expected Behavior

Next should return either the json data or throw error or warning that getServerSideProps is not valid.

To Reproduce

// index.js

const Page = () => {
  return <div>
    <Link href="/page"><a>Page</a></Link>
  </div>;
} 

export default Page;

// page.js

export const getServerSideProps = undefined;

const Page = () => {
  return <div>
    page
  </div>;
} 

export default Page;

If user clicks on the link, next tries to fetch data and /_next/data/development/page.json returns html.

Start with the index.js and page.js reproduction, then inspect handling of the /_next/data/development/page.json request when getServerSideProps is undefined. Confirm the behavior with next start on Next.js 12.0.7; done means the request returns valid JSON or a clear validation error rather than HTML.