fix(nushell): avoid nil root_dir #3861

From #3611 :

> nushell 0.102 made some changes that a nil root_dir will cause lsp server to exit.

Author: Bahex

lsp/nushell.lua

@@ -6,5 +6,7 @@
 return {
   cmd = { 'nu', '--lsp' },
   filetypes = { 'nu' },
-  root_markers = { '.git' },
+  root_dir = function(bufnr, on_dir)
+    on_dir(vim.fs.root(bufnr, { '.git' }) or vim.fs.dirname(vim.api.nvim_buf_get_name(bufnr)))
+  end,
 }