fix return value

weed/shell/command_volume_vacuum.go

@@ -34,7 +34,7 @@ func (c *commandVacuum) Do(args []string, commandEnv *CommandEnv, writer io.Writ
 	collection := volumeVacuumCommand.String("collection", "", "vacuum this collection")
 	volumeId := volumeVacuumCommand.Uint("volumeId", 0, "the volume id")
 	if err = volumeVacuumCommand.Parse(args); err != nil {
-		return
+		return nil
 	}
 
 	if err = commandEnv.confirmIsLocked(args); err != nil {