Allow tree data providers to return null items

[msujew]

What it does

Closes #13001

Some extensions return falsy values in their tree data providers. VSCode first filters the results by using coalesce. This change aligns Theia to the behavior of vscode to also allow falsy values.

How to test

1. Install the SonarLint extension from open-vsx
2. Open the SonarLint Issues Location view in the explorer panel
3. Assert that the welcome view info is shown and no error is logged on the backend.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

I can confirm the changes work as expected, and fixes the error 👍