Fix phpstan/phpstan#13674: infer non-empty-list after isset($list[$i])

[phpstan-bot]

This PR fixes phpstan/phpstan#13674

When isset($arr[$dim]) is used with a non-constant dimension (e.g. a variable $i), PHPStan now correctly narrows the array type to non-empty-array/non-empty-list.

Previously, isset($list[$i]) would narrow the dim type but not the array itself. Now, if both the PHPDoc type and native type are arrays, the array is narrowed to non-empty in the truthy branch.

The narrowing is guarded by checking that the native type is also an array, which prevents issues with template types in loop scope merging where the native type is mixed.

Examples

/** @param list<int> $list */
function foo(array $list, int $i): void {
    if (isset($list[$i])) {
        // Before: list<int>
        // After:  non-empty-list<int>
    }
}

/** @param array<string, int> $arr */
function bar(array $arr, string $key): void {
    if (isset($arr[$key])) {
        // Before: array<string, int>
        // After:  non-empty-array<string, int>
    }
}