fix: fallback action path for todo actions.

[boydaihungst]

Sometimes action.ctx.root="", so open and todo will use path="/filename". This will fallback to use action's buffer parent folder.

For example a single example.py file without project will return this action:

{
  _is_condition = <function 1>,
  command = "add_todo__todo_",
  condition = ".*",
  ctx = <1>{
    buf = 16,
    bufname = "/home/huyhoang/a.py",
    col = 0,
    edit = <table 1>,
    filetype = "python",
    line = "",
    range = {
      ["end"] = <2>{
        character = 0,
        line = 0
      },
      rc = { 0, 0, 0, 0 },
      start = <table 2>
    },
    root = "",
    row = 0,
    ts_range = { 1, 0, 1, 0 },
    ts_type = "module",
    win = 1000,
    word = "",
    <metatable> = {
      __index = <function 2>
    }
  },
  desc = "",
  fn = <function 3>,
  group = "Todo",
  name = "Add Todo",
  title = "Add Todo (todo)"
}