Make sure that `ocannl_log_level=nonempty_entries` doesn't output too little

[lukstafi]

Entries that are definitely non-empty, but do not have any values logged at their level, are skipped -- only their children subtrees are logged. This is a reasonable design for a log level but is unexpected given the log level name, and we definitely want the more generous log level too.

[lukstafi]

Whether to output empty entries should be decided based on %track (yes), %debug (?), %diagn (no), not on a log level.

[lukstafi]

This is fixed now with migration to explicit verbosity levels (there's no "nonempty entries" log level), and using the extension entry point type for filtering empty entries (most recent commit).