Fix harmless error message in logs when ScanSchedule set was "0"

Message:
`ERRO[0000] Error scheduling periodic scan                error="expected exactly 5 fields, found 1: [0]"`

cmd/root.go

@@ -63,7 +63,7 @@ func runNavidrome() {
 	if schedule != "" {
 		go schedulePeriodicScan(schedule)
 	} else {
-		log.Warn("Periodic scan is DISABLED", "schedule", schedule)
+		log.Warn("Periodic scan is DISABLED")
 	}
 
 	if err := g.Run(); err != nil {

conf/configuration.go

@@ -158,6 +158,7 @@ func validateScanSchedule() error {
 		}
 	}
 	if Server.ScanSchedule == "0" || Server.ScanSchedule == "" {
+		Server.ScanSchedule = ""
 		return nil
 	}
 	if _, err := time.ParseDuration(Server.ScanSchedule); err == nil {