message: fix: don't add useless null value to the tree
  `branch_attrs.mutations.nuc` = `null` was added to the tree when there was not a value already there. This fixes it.
diff: @@ -196,7 +196,13 @@ namespace Nextclade {
  std::vector<NucleotideSubstitution> nucleotideMutations() const {
  ensureIsObject();
  - auto nucMutsArray = j[json_pointer{"/branch_attrs/mutations/nuc"}];
  + const auto path = json_pointer{"/branch_attrs/mutations/nuc"};
  +
  + if (!j.contains(path)) {
  + return {};
  + }
  +
  + auto nucMutsArray = j.at(path);
  if (!nucMutsArray.is_array()) {
  // TODO: throw an exception
  }
  