fix(eslint-plugin): [no-extra-non-null-assertion] false positive with non-nullable computed key

[ddubrava]

Fixes #2732

function foo(key: string | null) {
  const obj = {};
  return obj?.[key!];
}

function foo(key: string | null) {
  const obj = {};
  return obj?.[a!?.key!];
}

function foo(key: string | null) {
  const obj = {};
  return obj!?.[key!];
}

Some extra code tests that were used in AST to check the solution

[typescript-eslint]

Thanks for the PR, @ddubrava!

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]

LGTM - nice and simple 😄 thanks a lot for the contribution!