Empty call costs in session should not be saved since they break

sessionmanager/session.go

@@ -160,6 +160,9 @@ func (s *Session) SaveOperations() {
 	go func() {
 		for _, sr := range s.sessionRuns {
 			engine.Logger.Debug(fmt.Sprintf("Saving operations for session %v, runId: %s", s, sr.runId))
+			if len(sr.callCosts) == 0 {
+				break // There are no costs to save, ignore the operation
+			}
 			firstCC := sr.callCosts[0]
 			for _, cc := range sr.callCosts[1:] {
 				firstCC.Merge(cc)