fix(eslint-plugin): [no-unnecessary-condition] fix false positive when checking indexed access types

[uhyo]

PR Checklist

• Addresses an existing open issue: fixes Bug: [no-unnecessary-condition] False positive Checking Lookup Types #6264
• That issue was marked as accepting prs
• Steps in Contributing were taken

Overview

This PR fixes the issue that the no-unnecessary-condition rule considers unconstrainted indexed access tyoes (Obj[Key]) as always truthy, leading to false positives.

export function get<Obj, Key extends keyof Obj>(obj: Obj, key: Key) {
  const value = obj[key];
  if (value) {
//    ^^^^^
//    This is not unnecessary, but flagged as error
    return value;
  }
  throw new Error('BOOM!')
}

get({ foo: null }, "foo");

The fix is as suggested in #6264 (comment) by @bradzacher -- just updating the flag seems to work fine.

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[typescript-eslint]

Thanks for the PR, @uhyo!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[bradzacher]

thanks for this!