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.

A state test loads {} and a file missing each section in turn, and
checks that every section is an empty map. A watcher test loads {}, and
a file without the certificates section, runs the first check against
live DNS, and checks that it saved domain, hostname, port and
certificate entries.

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 stop before the first scan completes sends no startup
   notification; a scan that ran to the end still sends it (closes #271).
 - 2026-10-13: a nameserver leaving a domain's delegation sends only NS Change;