rg scanner doesn't work on Windows

The recent changes which introduced `directory-form` in the rg scanner broke magit-todos on Windows.

With the default settings `magit-todos-list` starts an rg command, which looks like does it's job but the process keeps running and TODOs aren't inserted in the buffer. With a custom scanner that leaves `:directory-form` empty but otherwise is identical to the default one, everthing works (and results have a leading `./`).

As far as I can tell, the default scanner runs `rg ./`, while a scanner with empty `:directory-form` runs `rg ` (without the path at the end). In a shell both commands work, and their output differs in the leading `./`.

On macOS with the same config works without any issues. All testing done with Emacs 29.1 and latest MELPA packages.