fix: ensure TOML serialization in show() does not break it

[mortenpi]

If we happen to get nothings or other values that TOML.jl can not serialize in the the DataSet's .conf, the show() method will break printing in the REPL. So let's get rid of all problematic values. We don't care here that it is round-trippable -- this is just for a pretty show method.

In addition, there TOML.print's to_toml does not handle e.g. unsupported Dict keys. So we also just put a catchall try-catch around it and print something sensible, so that the users wouldn't be presented with a big show method stacktrace.