fix(irtt): treat blank HMAC keys as unauthenticated

README.md

@@ -76,3 +76,4 @@ config reflector
 Use `list files '/path/to/reflectors.csv'` instead of `option files 'none'` to
 combine CSV files with inline reflectors. Omitting `files` keeps the default CSV;
 UCI discards empty string options, so `option files ''` does not disable it.
+Absent or blank `irtt_hmac` values use unauthenticated IRTT sessions.