Before You File a Bug Report Please Confirm You Have Done The Following...

• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I have searched for related issues and found none that matched my issue.
• I have read the FAQ and my problem is not listed.

Playground Link

https://typescript-eslint.io/play/#ts=5.7.2&fileType=.ts&code=CYUwxgNghgTiAEYD2A7AzgF3gWygTwCMQBBGGALnkxgEsUBzAbQF14AfeAVxVADM6QwANwAoUJFgJk6LLApUMtBi1EiA9Gvi8oNCGkRIy4DBDwjeh%2BAAppmeDQwhs9lPDkBKeAG8AviPWaaAAWSJwQwFo6EAA0VFDYCADuDkHwAAYAKizs8ChhEGnmljaodg5OLjj4RKQwnr7%2BGvAADkiOKBg0UBCmVJz09CCYalCcGEj8AB7w4zNBCBY9SMkMAPxFMNa2WOXOE1WEJGTwq6vwLPV%2BQA&eslintrc=N4KABGBEBOCuA2BTAzpAXGUEKQAIBcBPABxQGNoBLY-AWhXkoDt8B6Jge1oDMPpbmtAIbRoQwuiiJRfSODABfEAqA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylBm3TgwAXxCSgA&tokens=false

Repro Code

declare const maybeArr: string[] | undefined;
declare const arr: string[];

// fails correctly
for (const item in arr) {}

// should fail, same with `T[] | null`
for (const item in maybeArr) {}

// potentially suggest/autofix to the following?
for (const item of maybeArr ?? []) {}

ESLint Config

module.exports = {
  parser: "@typescript-eslint/parser",
  rules: {
    "@typescript-eslint/no-for-in-array": "error",
  },
};

tsconfig

{
  "compilerOptions": {
    "strictNullChecks": true
  }
}

Expected Result

I expected the rule to report, as it doesn't make sense to iterate null or undefined. A fixer would be nice (in general, not just for this special case) since most of the time, the intent is to use for-of.

Actual Result

The rule fails to catch these cases.

Additional Info

Currently, the rule checks specifically for an array or union of arrays. I think this can be expanded to a union with an array and potentially null and undefined.