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

README.md

@@ -662,6 +662,10 @@ A port entry's `hostnames` lists every name that resolves to its address,
 domains included. A port entry in the older format, with one `hostname` instead
 of the `hostnames` list, loads as a list of that one name.
 
+A state file that leaves out any of `domains`, `hostnames`, `ports` and
+`certificates`, or holds `null` for one, loads with that section empty. A file
+holding `{}` loads as empty state.
+
 ---
 
 ## Entrypoints