nlohmann's JSON libraries in version 2.x used to strip bytes that
aren't valid UTF-8 from strings (e.g. service/station names in MPEG
transport streams) before outputting the JSON data structures. With
MKVToolNix v24 that bundled library was updated to v3 which now throws
an exception on such invalid data.
Therefore mkvmerge now takes care of replacing invalid bytes with
placeholder characters itself before passing the strings to nlohmann's
JSON library.
Fixes#2327.
