grace: fix empty copyOut (#97)

* fix: remove empty copyOut

* reserve capacity for copyOut

---------

Co-authored-by: Yang Gao <6821729+criyle@users.noreply.github.com>

cmd/go-judge-shell/grpc.go

@@ -181,7 +181,7 @@ func convertPBCopyIn(copyIn map[string]model.CmdFile) map[string]*pb.Request_Fil
 }
 
 func convertPBCopyOut(copyOut []string) []*pb.Request_CmdCopyOutFile {
-	rt := make([]*pb.Request_CmdCopyOutFile, len(copyOut))
+	rt := make([]*pb.Request_CmdCopyOutFile, 0, len(copyOut))
 	for _, n := range copyOut {
 		optional := false
 		if strings.HasSuffix(n, "?") {