Fix picker new_path_prompt throwing "file exists" when saving (#21080)

Fix for getting File exists "os error 17" with `"use_system_path_prompts": false,`

This was reproducible when the first worktree is a non-folder worktree
(e.g. setting.json) so we were trying to create the new file with a path
under ~/.config/zed/settings.json/newfile.ext

Co-authored-by: Conrad Irwin <conrad@zed.dev>