message: Fix lint_waiver which was comparing absl::string_view with nullptr.
diff: @@ -281,7 +281,7 @@ static absl::Status WaiveCommandHandler(
  }
  }
  - if (rule == nullptr) {
  + if (rule.empty()) {
  return WaiveCommandError(token_pos, waive_file,
  "Invalid rule: ", val);
  }
  @@ -349,7 +349,7 @@ static absl::Status WaiveCommandHandler(
  if (!location_match) return absl::OkStatus();
  // Check if everything required has been set
  - if (rule == nullptr) {
  + if (rule.empty()) {
  return WaiveCommandError(token_pos, waive_file,
  "Insufficient waiver configuration");
  }
  @@ -417,7 +417,7 @@ absl::Status LintWaiverBuilder::ApplyExternalWaivers(
  const std::set<absl::string_view>& active_rules,
  absl::string_view lintee_filename, absl::string_view waiver_filename,
  absl::string_view waivers_config_content) {
  - if (waivers_config_content == nullptr) {
  + if (waivers_config_content.empty()) {
  return absl::Status(absl::StatusCode::kInternal,
  "Broken waiver config handle");
  }
  