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