message: fix: don't report empty PCR primer changes
diff: @@ -61,9 +61,11 @@ namespace Nextclade {
  substitutionsSelected.push_back(mut);
  }
  + if (!substitutionsSelected.empty()) {
  result.emplace_back(PcrPrimerChange{.primer = primer, .substitutions = std::move(substitutionsSelected)});
  }
  }
  + }
  return result;
  }
  }// namespace Nextclade
  