workspace.workspaceFolders API should return undefined when no folder is open

[azatsarynnyy]

Bug Description:

workspace.workspaceFolders Plugin API should return undefined when no folder is open. But it returns an empty array. It leads to the failure of some VS Code extensions that rely on undefined value.

Steps to Reproduce:

1. Put the test plugin into the plugins folder and run Theia.
2. Call Hello World command from the Commands Palette when no workspace folder is opened.
3. It shows the message ws api returns 0 workspace folders, but has to show no workspace folders as it is in VS Code.

Additional Information

• Operating System:
• Theia Version: 1.6.0

[azatsarynnyy]

An example of the issue in VS Code extension redhat-developer/vscode-quarkus#310 (comment)