Null workspace folder not working with untitled documents

[renkun-ken]

Create an untitled document which has uri untitled:Untitled-1 in a null workspace folder (e.g. not opening a folder in , vscode), then root-path related providers will crash the language server.

[2021-07-24 10:56:33.164] parse_callback called: {
  "uri": "untitled:Untitled-1",
  "version": 13
}

[2021-07-24 10:56:33.165] deliver:  ["Response", "Message", "R6"]

[2021-07-24 10:56:33.166] deliver:  ["Response", "Message", "R6"]

[2021-07-24 10:56:33.166] deliver:  ["Response", "Message", "R6"]

[2021-07-24 10:56:33.167] deliver:  ["Response", "Message", "R6"]

[2021-07-24 10:56:33.167] deliver:  ["Response", "Message", "R6"]

[2021-07-24 10:56:33.168] Error: argument is of length zero
Call: if (!is_absolute_path(start)) {
    start <- path_norm(path(getwd(), start))
}
Stack trace:
1: fs::path_abs(source_file, self$rootPath)
2: parse_callback(self, uri, version, result)
3: private$callback(result)
4: task$check()
5: self$parse_task_manager$check_tasks()
6: self$process_events()

[2021-07-24 10:56:33.169] exiting

[Error - 10:56:33 AM] Connection to server got closed. Server will not be restarted.
[Error - 10:56:33 AM] Request textDocument/documentLink failed.
Error: Connection got disposed.
    at Object.dispose (/Users/ken/.vscode-insiders/extensions/ikuyadeu.r-2.1.0/dist/extension.js:2:1447696)
    at Object.dispose (/Users/ken/.vscode-insiders/extensions/ikuyadeu.r-2.1.0/dist/extension.js:2:1527935)
    at b.handleConnectionClosed (/Users/ken/.vscode-insiders/extensions/ikuyadeu.r-2.1.0/dist/extension.js:2:1528148)
    at b.handleConnectionClosed (/Users/ken/.vscode-insiders/extensions/ikuyadeu.r-2.1.0/dist/extension.js:2:1588679)
    at t (/Users/ken/.vscode-insiders/extensions/ikuyadeu.r-2.1.0/dist/extension.js:2:1526237)
    at invoke (/Users/ken/.vscode-insiders/extensions/ikuyadeu.r-2.1.0/dist/extension.js:2:1449313)
    at a.fire (/Users/ken/.vscode-insiders/extensions/ikuyadeu.r-2.1.0/dist/extension.js:2:1450074)
    at X (/Users/ken/.vscode-insiders/extensions/ikuyadeu.r-2.1.0/dist/extension.js:2:1436954)
    at invoke (/Users/ken/.vscode-insiders/extensions/ikuyadeu.r-2.1.0/dist/extension.js:2:1449313)
    at a.fire (/Users/ken/.vscode-insiders/extensions/ikuyadeu.r-2.1.0/dist/extension.js:2:1450074)
    at h.fireClose (/Users/ken/.vscode-insiders/extensions/ikuyadeu.r-2.1.0/dist/extension.js:2:1461542)
    at Socket.<anonymous> (/Users/ken/.vscode-insiders/extensions/ikuyadeu.r-2.1.0/dist/extension.js:2:1462327)
    at Socket.emit (events.js:327:22)
    at TCP.<anonymous> (net.js:673:12)