Protocol handler: handler not called if no window open

[weinand]

testing #48427 on macOS:

• create an extension that registers a protocol handler
• install it in VS Code Insiders
• verify that it works when running something like open vscode-insiders://andreweinand.protocol-handler/foo from the command line
• close all windows of VS Code
• run open vscode-insiders://andreweinand.protocol-handler/foo again

Observe: nothing happens