Add localization for empty navigator

[msujew]

What it does

Removes a long-standing TODO about localizing the message for empty multi-root workspaces.

How to test

1. Create an empty workspace file:

{
  "folders": [],
  "settings": {}
}

2. Open the workspace.
3. Observe that the message in the widget is correctly localized

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]

@msujew I believe the use-case might be incorrect, the renderEmptyMultiRootWorkspace method applies when a multi-root workspace with no roots is opened:

test.theia-workspace

{
  "folders": [
  ],
  "settings": {}
}

In such a case there is a bug with the implementation:

[msujew]

@vince-fugnitto Thanks for the hint :)

I've updated the changes to be a bit less ambitious and updated the testing steps as well.

[vince-fugnitto]

LGTM 👍

• confirmed the rendering is correct
• confirmed the content is properly localized