Stop causing errors by parsing empty strings as time.Time

We encode the zero time as an empty string, so we should do the
reverse when decoding.

Otherwise time.Parse() returns an error, which is created on the heap,
causing extra garbage-collection load.

report/metrics.go

@@ -238,6 +238,9 @@ func renderTime(t time.Time) string {
 }
 
 func parseTime(s string) time.Time {
+	if s == "" {
+		return time.Time{}
+	}
 	t, _ := time.Parse(time.RFC3339Nano, s)
 	return t
 }