message: fix: avoid potential dereference of nullopt
  If there is no open range by the end of the loop, then `begin` might be `nullopt`. Then it's illegal to dereference it in the body of this conditional. I add an additional check to see if `begin` contains a value.
diff: @@ -65,8 +65,8 @@ namespace Nextclade {
  }
  }
  - // Terminate the last range with the last deletion
  - if (!privateDeletionsSorted.empty()) {
  + // Terminate the last range if any (there is an open range if `begin` is set)
  + if (!privateDeletionsSorted.empty() && begin) {
  const auto end = privateDeletionsSorted.back().pos;
  const auto length = end - *begin;
  deletionRanges.emplace_back(
  