fix: resolve race condition in get_dir_file_info() when files are deleted concurrently

[gr8man]

Description
When scanning directories, get_dir_file_info() could encounter a race condition if a file was deleted right after being read by readdir(). In this scenario, get_file_info() would return null, causing an incomplete entry (lacking server_path or other keys) to be added to the result array.

This PR adds a null check to gracefully ignore files deleted mid-scan, which also prevents broken entries from being created when passing directories with $topLevelOnly = true. This fixes random test failures observed in the Commands component. A unit test has been added to ensure the correct behavior.

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