fix(blog): Fix SLO by using optional vector

apps/k8s01/blog/slo.yaml

@@ -11,7 +11,7 @@ spec:
       description: "Blog: SLO based on availability for HTTP request responses."
       sli:
         events:
-          errorQuery: sum(rate(nginx_ingress_controller_requests{exported_namespace="blog",ingress="blog",status=~"(5..|429)"}[{{.window}}]))
+          errorQuery: sum(rate(nginx_ingress_controller_requests{exported_namespace="blog",ingress="blog",status=~"(5..|429)"}[{{.window}}])) OR vector(0)
           totalQuery: sum(rate(nginx_ingress_controller_requests{exported_namespace="blog",ingress="blog"}[{{.window}}])) > 0 OR vector(1)
       alerting:
         name: BlogHighErrorRate