This PR doesn't change behavior, but simply adds a case to a test to
make sure we continue to scan any given worktree for project types, if
one was not found prior.

Also updates `detect_project_types`'s return type to an `Option` so we
can differentiate in tests between the case where we skip a worktree
that previously had project type events sent and the case where we
simply found no project types to report.

Release Notes:

- N/A