Address multisite PHP Warning: Undefined array key 0 in Has_Multiple_Admins.php line 93

## Bug Description

As reported by [a user in the support forums](https://wordpress.org/support/topic/php-warning-undefined-array-key-0-in-has_multiple_admins-php-line-93-multisite/), in multisite environments, it's possible to encounter a PHP warning for PHP 8.2 and upwards on a multisite environment when returning a list of returns an array of usernames via `superadmins()` if that array has been filtered or manipulated by another plugin.

See the function and associated actions in [Has_Multiple_Admins.php](https://github.com/google/site-kit-wp/blob/e1e7f298e4acd8ac7cd0c9735b6f1802598d0c18/includes/Core/Authentication/Has_Multiple_Admins.php#L82).

## Screenshots

## Additional Context

- PHP Version:
- OS: [e.g. iOS]
- Browser: [e.g. chrome, safari]
- Plugin Version: [e.g. 22]
- Device: [e.g. iPhone6]

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## Steps to Reproduce

- Set up a WordPress multisite environment with Site Kit active.
- Add a temporary mu-plugin that forces the multisite super admin list to be sparse, for example:

```php
'admin',
);
} );
```

- Open the Site Kit setup splash screen at `wp-admin/admin.php?page=googlesitekit-splash` while logged in as an admin user.
- Let the page load fully so it requests `GET /wp-json/google-site-kit/v1/core/site/data/connection`.
- That request reaches `Has_Multiple_Admins::get()` and triggers the warning when index `0` is missing.
- Observe the PHP warning: `Undefined array key 0` in `includes/Core/Authentication/Has_Multiple_Admins.php`.

## Acceptance Criteria

- There should be no php warning when the filtered super admin array is non-zero based index.

## Implementation Brief

- [ ] Update `includes/Core/Authentication/Has_Multiple_Admins.php` so the multisite branch does not assume the first super admin is stored at array index `0`.
- Normalize the `get_super_admins()` result or safely read the first login value before calling `get_user_by()`, so the multisite branch works with sparse or otherwise non-sequential array keys.
- Only add the resolved user ID to the exclusion list when the lookup returns a valid user object.

### Test Coverage

- Update `tests/phpunit/integration/Core/Authentication/Has_Multiple_AdminsTest.php`, add a regression case where the super admin list is sparse or otherwise non-sequentially keyed.
- Assert that the warning condition is avoided and the resulting admin count still matches the expected total.

## QA Brief

*

## Changelog entry

*

Start with includes/Core/Authentication/Has_Multiple_Admins.php around the multisite branch and review tests/phpunit/integration/Core/Authentication/Has_Multiple_AdminsTest.php. Run the existing test before adding a regression case with a sparse super admin list. Done means the case produces no PHP warning and the resulting admin count remains correct.

Written by the indexing model from the issue text.