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

internal/watcher/watcher_test.go

@@ -477,6 +477,54 @@ func TestDomainPortAndTLSChecks(t *testing.T) {
 	}
 }
 
+// TestFirstCheckAfterLoadingFileWithoutSections loads a state file
+// holding {}, and one that leaves out the certificates section, and runs
+// the first check. It saves domain, hostname, port and certificate
+// entries; a section loaded as nil made it panic.
+func TestFirstCheckAfterLoadingFileWithoutSections(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		file string
+	}{
+		{"no sections", `{}`},
+		{"no certificates", `{"domains": {}, "hostnames": {}, "ports": {}}`},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			cfg := defaultTestConfig(t)
+			cfg.Domains = []string{testSmallDomain}
+
+			_, deps := runChecks(t, cfg, func(deps *testDeps) {
+				err := os.WriteFile(cfg.StatePath(), []byte(tt.file), 0o600)
+				if err != nil {
+					t.Fatalf("writing the state file: %v", err)
+				}
+
+				err = deps.state.Load()
+				if err != nil {
+					t.Fatalf("loading the state file: %v", err)
+				}
+			})
+
+			snap := deps.state.GetSnapshot()
+			if len(snap.Domains) == 0 || len(snap.Hostnames) == 0 ||
+				len(snap.Ports) == 0 || len(snap.Certificates) == 0 {
+				t.Errorf(
+					"saved %d domain, %d hostname, %d port and %d "+
+						"certificate entries, want at least one of each",
+					len(snap.Domains), len(snap.Hostnames),
+					len(snap.Ports), len(snap.Certificates),
+				)
+			}
+		})
+	}
+}
+
 func TestNSChangeDetection(t *testing.T) {
 	t.Parallel()