Error in empty repos

It's obvious that magit-todo's can't to anything in an empty repo :) Since we get `error in process sentinel` this sounds like an unhandled case so far, though.

I just created a new repo via `magit-init`:

```
error in process sentinel: Wrong type argument: (or eieio-object cl-structure-object oclosure), nil [2 times]
error in process sentinel: magit-todos--async-when-done: magit-todos--async-when-done: Process "magit-todos--scan-with-rg" failed with exit code 2. Output:"rg: No files were searched, which means ripgrep probably applied a filter you didn't expect.
Running with --debug will show why files are being skipped.
"
```