fix(eslint-plugin): [no-unnecessary-condition] improve checking optional callee

[yeonjuan]

PR Checklist

• Addresses an existing open issue: fixes Bug: [no-unnecessary-condition] Improper checking of optional chaining of defined method return value #7912
• That issue was marked as accepting prs
• Steps in Contributing were taken

Overview

This pr improves unnecessary optional chaining checking of no-unnecessary-condition.
In below example, optional chain in y()?. is unnecessary because y does not return nullable value.
As described in #7912, the current no-unnecessary-condition does not check for the following case.

type T1 = { y: () => number } | null;
declare const x1: T1;
x1?.y()?.toExponential();

I added isCallExpressionNullableOriginFromCallee function.
This function takes a CallExpression node as an argument. It returns true if the call expression's nullable type is origin from callee otherwise returns false.

// isCallExpressionNullableOriginFromCallee will return true for given call expression `y()`
type T1 = { y: () => number } | null;
declare const x1: T1;
x1?.y()?.toExponential();
//    ^^^ y()'s nullable type is origin from T1's nullable type

// isCallExpressionNullableOriginFromCallee will return false for given call expression `y()`
type T1 = { y: () => number | null } | null;
declare const x1: T1;
x1?.y()?.toExponential();
//    ^^^ y()'s nullable type is origin from it's own definition. (() => number | null)

[typescript-eslint]

Thanks for the PR, @yeonjuan!

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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[JoshuaKGoldberg]

Tricky 😄 but this look great. Thanks for working on it!