Don't create "." directory in queries/tests tree view

[shati-patel]

The createDirectory method that we use for populating the Queries Panel and Test Explorer tree view had a slight bug. If a query/test was directly under the root directory, we'd unnecessarily create an empty subdirectory called .
(instead of directly adding the query/test).

This PR prevents us from creating a directory when there's none to create. Thanks to @robertbrignull for the fix 🙇🏽‍♀️

(see internal linked issue for more details)

Checklist

N/A—feature-flagged for internal development only 🎏

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.