Handle empty dashboard metrics safely

internal/service/safety_test.go

@@ -2,6 +2,7 @@ package service
 
 import (
 	"context"
+	"encoding/json"
 	"os"
 	"path/filepath"
 	"testing"
@@ -11,6 +12,17 @@ import (
 	"git.casaderoll.de/michael/urbm/internal/store"
 )
 
+func TestEmptyBackupMetricsEncodeAsJSONArray(t *testing.T) {
+	s := &Service{}
+	encoded, err := json.Marshal(s.BackupMetrics())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(encoded) != "[]" {
+		t.Fatalf("empty backup metrics JSON = %s", encoded)
+	}
+}
+
 func TestRepositoryLockHonorsContextCancellation(t *testing.T) {
 	s := &Service{repositoryLocks: map[string]chan struct{}{}}
 	repo := model.Repository{Type: model.RepositoryLocal, Location: "/repo"}