writeYaml encodes null values as "null". That's only legal in English according to [the YAML specification|https://yaml.org/type/null.html,] and some processors like Perl's YAML::Syck don't deal with it well. It would be most portable if the canonical representation was used: "~". Although the empty "" or shorthand "!!null" would probably also be good.
I think this could be fixed by a custom Representer class that sets the nullRepresenter to use "~" instead of "null", although arguably this should be fixed in the RepresentNull class itself. I don't see any utility in using "null" over "~".