message: Fix bug when writing StorableResources
  Previously StorableResources was only written if `>` was true, which was
  only true if every member element compared `>` with 0. That meant if any
  resource type was 0, the entire set would not be saved.
diff: @@ -48,13 +48,13 @@ namespace
  }
  const auto& production = structure->production();
  - if (production > StorableResources{ 0 })
  + if (!production.isEmpty())
  {
  writeResources(structureElement, production, "production");
  }
  const auto& stored = structure->storage();
  - if (stored > StorableResources{ 0 })
  + if (!stored.isEmpty())
  {
  writeResources(structureElement, stored, "storage");
  }
  