message: Fix empty hlo category to show as unknown.
diff: @@ -64,9 +64,10 @@ void DeviceOpMetricsDbBuilder::EnterOp(
  DCHECK_GE(time_ps, self_time_ps);
  OpMetrics* op_metrics = LookupOrInsertNewOpMetrics(program_id, name);
  if (op_metrics->category().empty())
  - op_metrics->set_category(std::string(category));
  + op_metrics->set_category(category == kUnknownOp ? "unknown"
  + : string(category));
  if (op_metrics->provenance().empty())
  - op_metrics->set_provenance(std::string(provenance));
  + op_metrics->set_provenance(string(provenance));
  op_metrics->set_occurrences(op_metrics->occurrences() + occurrences);
  op_metrics->set_time_ps(op_metrics->time_ps() + time_ps);
  op_metrics->set_self_time_ps(op_metrics->self_time_ps() + self_time_ps);
  