state: a state file without a section loads it empty (closes #267)

Load fills in each of domains, hostnames, ports and certificates that
the file leaves out or holds as null with an empty map. Before, such a
section loaded as a nil map, and the first check's write into it
panicked, so a state file holding {} stopped the watcher at every
start.

The test loads {} and a file missing each section in turn, then saves
an entry in every section as the first check does; it does not run a
check, which would look names up in live DNS.

Model: opus-5-5

TODO.md

@@ -19,6 +19,8 @@ trial run of the finished image: https://git.eeqj.de/sneak/dnswatcher/issues/149
 
 # Completed Steps
 
+- 2026-10-13: a state file holding `{}`, or leaving out a section, loads with
+  that section empty; the first check no longer crashes on it (closes #267).
 - 2026-10-13: a save syncs the state file to disk before renaming it into place,
   so a crash cannot leave it empty and stop the next start (closes #264).
 - 2026-10-13: a `PORT` that cannot be bound stops startup with an error naming