fix: null as an array offset is deprecated, use an empty string instead

[ddevsr]

Description
Reference : https://github.com/codeigniter4/CodeIgniter4/actions/runs/20359640614/job/58502356892?pr=9847

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[michalsn]

While I agree with the approach taken in RulesTest.php, I'm not in favor of the proposed changes to FileLocator.

In FileLocator, null has a semantic meaning and should be preserved. The issue exists only in the caching layer (FileLocatorCached), not in the core logic (FileLocator) itself.

Given that, I would limit the fix to the caching layer and simply change the FileLocatorCached::locateFile() method:

$folderKey = $folder ?? '';

if (isset($this->cache['locateFile'][$file][$folderKey][$ext])) {
    return $this->cache['locateFile'][$file][$folderKey][$ext];
}

$files = $this->locator->locateFile($file, $folder, $ext);

$this->cache['locateFile'][$file][$folderKey][$ext] = $files;
$this->cacheUpdated                                 = true;

return $files;

[michalsn]

Hmm... I'm not sure what's happening with the Sanity Tests. Everything works fine locally, and the tests also pass on GitHub Actions: https://github.com/michalsn/CodeIgniter4/actions/runs/20376659038/job/58556861626

Maybe try to rebase?