** files.exclude filter should filter everything in the navigator

[kittaakos]

This is a follow-up issue of #4274. Most likely, nobody wants to filter all items from the navigator, so this is a minor defect.

{
    "files.exclude": {
        "**": true
    }
}

Current behavior:
All files with a leading . (dot) are visible in the navigator.

Expected behavior:
No files are visible in the navigator.