message: [gha][lbt] bug fix for error reporting when report empty
  Closes:
diff: @@ -145,7 +145,7 @@ jobs:
  // Read and check cluster test results
  let should_fail = false;
  let env_vars = process.env;
  - let body;
  + let body = '';
  const fsp = require('fs').promises;
  try {
  data = await fsp.readFile('report.json', 'utf-8');
  @@ -191,7 +191,7 @@ jobs:
  console.error("Failed to check infra error in CT output log.\n", err);
  }
  } else {
  - body += "\n Cluster Test failed - test report processing failed.";
  + body = "Cluster Test failed - test report processing failed.";
  console.error(err);
  }
  body += " See https://github.com/libra/libra/actions/runs/${{github.run_id}}";
  