`if` does not guard `unkown` not `null` correctly

[JounQin]

Bug Report

🔎 Search Terms

if unkown null

🕗 Version & Regression Information

typescript@next

This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________

⏯ Playground Link

Playground link with relevant code

💻 Code

declare const error: unknown

if (error) {
  throw typeof error === 'object' && 'message' in error ? Object.assign(new Error(), error) : error
}

🙁 Actual behavior

Object is possibly 'null'.

🙂 Expected behavior

No error, if (error) has already ensured error will never be null.

[whzx5byb]

Duplicate of #28131

[JounQin]

Thanks.