Add the following two settings into your vscode settings.json:
"workbench.startupEditor": "welcomePageInEmptyWorkbench", "window.restoreWindows": "none",
^ Alternatively you can set the above settings from settings UI menu too.
This would ensure that you get the welcome page when opening an empty workbench. If in all cases you'd like to show welcome page then set it like so: "workbench.startupEditor": "welcomePage".
Important note
If you had opened a folder/workspace from command line in vscode, like say code ~/dev/myProj, then that session will always be persisted during restart of vscode. That's just how vscode works. So inorder to make the above settings work, you have to open the folder/file from vscode command palette(Ctrl+Shift+P) rather than from command line(or terminal).