Function emit 

pub fn emit(
    value: Value,
    format: Format,
    pretty: bool,
    null_as: Option<&str>,
) -> Result<String>

Converts the merged IR into format and serializes it.

null_as substitutes a string for every null rather than failing on one. It is honoured in the TOML arm and nowhere else: JSON can hold a null perfectly well, so there is nothing there for it to rescue and substituting anyway would corrupt a document that was never in trouble.

The null pre-check inside value::to_toml is then the only thing that can fail here, and it reports key paths alone — nothing about the inputs survives the merge for it to name.