Do not write build_error proto file if there are no build errors.

For successful builds, a blank build_error proto file is created.
This slows down the data processing part where it reads the build_error
file and there are no errors to be processed.

Bug: 142277430
Test: Did a successful build and checked if the build_error file
      was not available. Changed a source file to make the build
      failed and checked if the build_error file was available.
Change-Id: I300dffe32fb7f8bf984fa20ae368bdd02bf12992

ui/status/log.go

@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"os"
 	"strings"
 
 	"github.com/golang/protobuf/proto"
@@ -154,6 +155,7 @@ type errorProtoLog struct {
 }
 
 func NewProtoErrorLog(log logger.Logger, filename string) StatusOutput {
+	os.Remove(filename)
 	return &errorProtoLog{
 		errorProto: soong_build_error_proto.BuildError{},
 		filename:   filename,
@@ -178,11 +180,17 @@ func (e *errorProtoLog) FinishAction(result ActionResult, counts Counts) {
 }
 
 func (e *errorProtoLog) Flush() {
+	// Don't create the build error proto file if there is action errors.
+	if len(e.errorProto.ActionErrors) == 0 {
+		return
+	}
+
 	data, err := proto.Marshal(&e.errorProto)
 	if err != nil {
 		e.log.Printf("Failed to marshal build status proto: %v\n", err)
 		return
 	}
+
 	err = ioutil.WriteFile(e.filename, []byte(data), 0644)
 	if err != nil {
 		e.log.Printf("Failed to write file %s: %v\n", e.filename, err)